With all due respect . . . I will blow you out of the sky.1

I. Introduction: The Need for a Counter to Drones Operating Without Due Regard

Ever since the days of canvas sails and cannon fire, one of the primary missions of the U.S. Navy has been to maintain the freedom of the seas.2 To enforce and defend that freedom, the U.S. Navy is sworn to “fly, sail and operate wherever international law allows.”3 But the rise of drones4 and their lack of due regard for other vessels and aircraft5 is making this an increasingly dangerous mission for the U.S. Navy.

Imagine that you are on a U.S. Navy aircraft carrier in the Persian Gulf. Shortly after transiting the Strait of Hormuz,6 the carrier resumes normal peacetime flight operations. Suddenly, watchstanders spot an Iranian Shahed 129 drone7 off the port side of the carrier. Within the open water Exclusive Economic Zones (EEZ)8 of the Persian Gulf, the Iranian drone enjoys the same high seas freedoms, including overflight, that the aircraft carrier and its military aircraft do.9 As the crew watches, the drone moves within four nautical miles (NM) of the ship and begins to shadow its every move, presumably gathering intelligence on naval flight operations. Radio transmissions directing the drone to depart go unanswered and generate no responding action. The drone neither takes any hostile actions nor exhibits any indicators of hostile intent,10 but its presence inside the “stack”11 violates its obligation under international law to act with due regard for the safety and navigational rights of others. More importantly, not only is the drone impeding the carrier’s operations, it poses a significant danger to the lives of the Sailors and Airmen aboard the ship and its associated aircraft. In this gray area, the carrier commander must be able to avail him or herself of self-help countermeasures to protect the ship, including using force against the drone to “blow [it] out of the sky.”12

From the Strait of Hormuz to the South China Sea, U.S. warships face situations like the hypothetical above with increasing frequency and frustration, and will continue to do so. The advent of drones may be revolutionizing warfare, particularly the U.S. approach to warfare, but the United States no longer enjoys a monopoly on the use of unmanned systems.13 More than thirty nations possess, or are developing, armed drones, and dozens more, including some non-State actors, possess unarmed reconnaissance drones.14 Thus far, responsible nations like the United States operate their drones with due regard for the safety of navigation of other craft.15 Yet as drone proliferation continues, more and more U.S. naval commanders will find themselves in a gray area where action is needed to counter an adversary drone’s failure to act with due regard for the navigation and safety of other vessels and aircraft. However, in that situation, traditional self-defense is not warranted under international law because there has been no armed or demonstrated hostile intent. Inaction or acquiescence is not the answer. If “the history of warfare [is] a struggle between measures and countermeasures,”16 then an appropriate countermeasure is needed.

In such situations, necessity and common sense dictate that the ship use an appropriate level of force to counter the safety threat posed by the drone. Yet for most of the last century, international law disfavored the use of forcible countermeasures17 against an offending State.18 How then can the naval commander justify taking an action to protect the aircraft carrier in the hypothetical example above? By examining the nature of unmanned vehicles, how international law applies to such vessels vis-à-vis the obligation of due regard, and the character of acceptable uses of force not rising to the level of an “armed attack,” this article shows that international law must, and in fact already does, permit necessary and proportional forcible countermeasures. “The only thing necessary for the triumph of evil is for good men to do nothing.”19 So long as the intended countermeasures pose no threat to human life or the sovereignty of a foreign State, then a U.S. commander is free to employ forcible countermeasures to safeguard U.S. ships from the danger posed by a drone that, while not engaged in a hostile act or exhibiting indicators of hostile intent, is nevertheless failing to act with due regard by interfering with naval operations and posing a danger to safety, navigation, and the lives of U.S. Sailors and Airmen.

II. The Universal Obligation to Act with Due Regard

From a historic, common-sense standpoint, it is obvious that vessels in peacetime maritime encounters should act with due regard to each other’s safety and navigation in order to avoid collisions and other mishaps. At a macro-level under international law, a State is to have due regard to the rights, duties, and freedoms of other states when it exercises its own rights, duties, and freedoms.20 Yet “due regard” is an “elusive” phrase that is not clearly defined in international law.21 Nevertheless, “due regard” is a firm requirement under the Convention on the International Regulations for Preventing Collisions at Sea (COLREGS),22 the United Nations Convention on the Law of the Sea (UNCLOS),23 and other sources of international law.24

A. Due Regard Under the COLREGS

Under the COLREGS, which predates UNCLOS, “due regard” applies at the level of individual vessels. For instance, all vessels are required to exhibit due regard to the observance of “good seamanship”25 when taking action to avoid collisions between vessels.26 In practice, this means that all vessels shall, in order to not impede the passage of another vessel, take early action to allow sufficient sea-room for the safe passage of the other vessel.27 Of particular importance to the U.S. Navy, this includes a requirement for other vessels to keep out of the way of “a vessel restricted in her ability to manoeuvre (sic),”28 such as vessels engaged in the launch or recovery of aircraft.29 Vessels and their owners, masters, and crews are accountable for any neglect to comply with this basic requirement for due regard to all dangers of navigation and collision, or to any other ordinary practice of seamanship.30

This obligation applies to U.S. naval vessels the same as any other vessels. Though there are situations where the COLREGS require a vessel not to impede the passage or safe passage of another vessel, e.g., “a vessel restricted in her ability to manoeuvre,”31 such provisions do not give an absolute right to navigation at the expense of other vessels. All vessels, to include warships and other military vessels, are required to affirmatively take action to avoid collision.32 Even if another vessel ignores its own obligations under the COLREGS, an affected vessel remains obligated, if the circumstances of the case permit, to comply with the rules and exercise due regard for the danger of collision.33 This obligation is clear when applied to finite encounters between only two vessels, such as a U.S. warship altering course to avoid a commercial container ship. Yet at some point, when a vessel poses a continuing, or repeat, danger to navigation, such as the threat posed the unmanned aerial vehicle (UAV) in the above hypothetical, relying solely on the obligation of due regard under the COLREGS may not be appropriate, particularly when dealing with State vessels.

B. Due Regard Under UNCLOS

For at-sea interactions and issues between States, UNCLOS expanded on the precept of due regard. As a basic tenet of relations between sovereign States, UNCLOS requires that each State should fulfill its obligations in good faith and exercise its rights and freedoms in such a manner that does not constitute an “abuse of right.”34 In thirteen instances throughout the text of UNCLOS, this tenet translates to a requirement to act in due regard to the rights and/or interests of other States.35 Such respect for the right of other States to make use of the seas is an outgrowth of the traditional freedom of the seas that governed the “law of the high seas” for centuries prior to the development of UNCLOS.36 Just as “freedom of the sea” traditionally meant freedom of navigation,37 the substantive uses of “due regard” by UNCLOS principally require due regard for safety and navigation, and for the rights and duties of other States.38

Yet similar to the COLREGS, the drafters of UNCLOS declined to provide a clear definition of what it means to have “due regard.” Subsequent interpretations by international courts provide intentionally vague, situation-specific definitions that rest on the circumstances and the nature of the rights affected.39 Due regard is, in effect, a balancing test of the concurrent respective rights between States.40 It does not require due regard by one State while allowing the other to do as it wishes; “the extent of the regard required by the Convention will depend upon the nature [and importance] of the rights held . . ., the extent of the anticipated impairment, the nature and importance of the activities contemplated by the [other State], and the availability of alternative approaches.”41 This is illustrated by the reciprocal obligations of due regard between coastal and foreign States when a vessel conducts innocent or transit passage.42

In addition, UNCLOS recognizes the inherent dangers of the sea by requiring due regard for safety and navigation outside of any implied balancing test between the competing rights and interests discussed above. In Article 21, UNCLOS requires ships conducting innocent passage through the territorial sea of another State to comply with “all generally accepted international regulations relating to the prevention of collisions at sea.”43 Moreover, Article 39 specifically requires ships transiting an international strait to comply with generally accepted international regulations, procedures and practices for safety at sea, including the International Regulations for Preventing Collisions at Sea.”44

This emphasis on due regard and incorporation of the COLREGs highlights the universal nature of the international requirement, arising by treaty and as part of customary international law (CIL), to exercise due regard in ensuring the safe passage of vessels and in actively avoiding all dangers of navigation and collision. Lack of due regard is a threat, and failing to comply with this obligation, whether negligently, recklessly, or by design, constitutes an internationally wrongful act and a breach of international law.45

III. The Rise of Drone Warfare

Warfare conducted by remotely-controlled devices or semi-autonomous robots was once the exclusive province of science fiction writers and conspiracy theorists. But, over the course of the last several decades that fiction became a reality, and drone warfare is now poised to be a cornerstone of 21st century combat operations. Concomitant with that new status is the need to define the legal parameters within which drones must operate. Drones, both UAVs and unmanned maritime systems (UMS), like any other vessel or craft at sea, must comply with the international obligation to act with due regard for the safety and navigation of other craft.46

A. A Brief History of Unmanned Vehicles in Combat

Since the dawn of aerial warfare, mankind has sought increasingly innovative ways to expand the range, use, and lethality of remotely-operated munitions and devices.47 Even before the advent of winged aircraft, one of the earliest examples of unmanned “vehicles” in combat is the mid-19th century use by the Austrian military of explosive-laden balloons that were controlled by timed fuses.48 As far back as World War I various States began experimenting with small, radio-controlled aircraft that were essentially flying bombs,49 or that were designed for target practice.50 Reusable remotely-operated reconnaissance aircraft began to appear during the Cold War and on the battlefields of Vietnam.51 During the 1991 Gulf War, U.S. naval forces utilized remotely-piloted reconnaissance aircraft to help ensure accurate naval bombardment of entrenched Iraqi targets.52 This famously lead to several Iraqi combatants surrendering to one of the drones launched from the battleship USS Wisconsin (BB-64), a first in the history or warfare.53 Yet, it is the massive expansion of offensive drone operations in the conflicts following the September 11, 2001, terrorist attacks that refined and established drone warfare as a fixture of future conflicts.54 Instead of merely conducting reconnaissance of enemy positions, for the first time, drones were used in lieu of conventional military forces to conduct armed attacks against military targets.55

B. Use and Status of Modern Drones

With the United States at the forefront of drone use and tactics,56 unmanned vehicles completed the transition from remotely-piloted bombs to short-range, reusable surveillance aircraft, and finally to semi-autonomous unmanned vehicles used to target and strike enemy combatants from thousands of miles away. Modern drones are highly versatile; drones can be used for kinetic strikes, intelligence, surveillance, reconnaissance, targeting, search and rescue, and transportation of supplies.57 Yet, like any other revolution in warfare and military technology, e.g., bomber aircraft, submarines, nuclear weapons, etc., international law has lagged behind the advent of drone warfare and has been slow to adapt to this advanced technology.58

For UAVs, the lack of international regulations is largely immaterial. While there is no universal international legal definition of “aircraft,”59 conventional domestic definitions of “aircraft” are generally broad enough to include all forms of UAVs.60 Moreover, major military powers and drone operators such as the United States, United Kingdom, and Germany have specifically designated their UAVs as Military Aircraft,61 an international legal term of art denoting that subset of state-operated aircraft that are used for military purposes.62 Though they are neither universal nor binding, several recent international agreements, such as the Code for Unplanned Encounters (CUES) at Sea that was signed by the United States and twenty other nations, specifically define UAVs as military aircraft.63 Consequently, there appears to be emerging CIL64 treating UAVs operated by the military as Military Aircraft. With this designation, all manner of existing international legal regimes applying to State and Military Aircraft would apply to UAVs regardless of a UAV-specific treaty requiring such application.

In contradistinction, UMS65 present unique challenges in understanding the application of existing law.66 Not only do unmanned surface vehicles (USV) and unmanned underwater vehicles (UUV) execute the same wide variety of functions that their UAV cousins perform, but they must do so while facing “a unique challenge that other unmanned vehicles do not: the ocean.”67 As a result, UMS generally “need to be [more] autonomous, [better] able to withstand the rigors of the marine environment, and be among the most power-efficient vehicles designed.”68 Moreover, as seaborne devices and/or craft to which the law of the sea may apply, “the status of these systems is an important question, for it entails important rights and obligations.”69 Yet, at present, there are no UMS-specific international laws or treaties governing the status, design, or use of USVs and UUVs. Additionally, unlike UAVs, there is no international consensus on whether or not military UMS should be considered warships (i.e., the functional maritime equivalent of Military Aircraft), which would impose certain rights and responsibilities that come with such designation.70 This absence of applicable, UMS-specific law is glaring considering the significant use of UMS in future maritime security operations and naval warfare.71

However, that is not to say international law is wholly inapplicable to UMS; “despite the novelty of UMSs, states must apply the existing law to them in good faith.”72 There are basic requirements for all vessels that UMS must meet, such as the obligation to act with due regard.

C. Drones Must Act with Due Regard

The obligation to exercise due regard, and any resulting accountability for failure to comply, applies to drones the same as it does to any other vessel above, on, or below the surface of the ocean. Qualification as a vessel is an important distinction that provides certain navigational rights and obligations that are different than those provided to aircraft and non-vessels, e.g., torpedoes, navigational buoys, etc.73 Yet UNCLOS, the seminal legal regime governing navigational rights and responsibilities of ships at sea, does not define the terms “ship” or “vessel,” which it uses interchangeably.74 In interpreting such an undefined term, it is acceptable to leave such a definition up to previously established definitions arising from other international laws and regulations, such as the COLREGS.75 The COLREGS broadly define “vessel” to include “every description of water craft, including non-displacement craft, [wing-in-ground] craft and seaplanes, used or capable of being used as a means of transportation on water.”76

USVs and UUVs, regardless of how they are controlled or piloted, are water craft that fall within this broad definition of vessel under the COLREGS. The problem is that this is an emerging area of law and there is no international consensus on this point yet. There is obviously disagreement in the field, as at least one author questioned whether USVs and UUVs are vessels as they are not currently used as transportation in the traditional sense.77 However, every UMS transports some form of weapons system, sensor array, or other equipment that is utilized to complete its mission. The more generally supported viewpoint is that UMS are “vessels” and that the legal regimes of UNCLOS and the COLREGs provide an adequate and existing legal framework for UMS during peacetime military operations.78

Additionally, U.S. policy appears to agree with this viewpoint, as it defines USVs as “water craft” and UUVs as “underwater craft.”79 Although these terms do not have independent legal significance under any treaty or CIL, the United States asserts that USVs and UUVs enjoy sovereign immunity and possess the same navigational rights of other exclusively state-owned and operated ships and submarines under the law of the sea.80 Consequently, under the American view and the predominant international view, international navigational rights and obligations for vessels, to include the obligation of due regard arising from the COLREGS and UNCLOS, apply to UMS.

Similarly, UAVs are military aircraft81 that are considered to be “State Aircraft” within the meaning of the Convention on International Civil Aviation of 1944 (Chicago Convention).82 While the Chicago Convention does not specifically apply to State Aircraft,83 it does establish that State and Military Aircraft must have due regard for the safety of navigation of other aircraft.84 Moreover, as Military Aircraft, UAVs retain sovereign immunity and overflight rights under customary international law, as reflected in UNCLOS, for peacetime military operations.85 Accordingly, they have similar rights and obligations under UNCLOS and the COLREGS as UMS do.

Therefore, the mandate of due regard from the COLREGS and UNCLOS applies to aerial and maritime drones exactly the same as it applies to any other vessel or aircraft. As a result, if these drones violate international law and endanger U.S. ships, the United States is entitled to take appropriate defensive countermeasures in response.

IV. Countermeasures and the Use of Force at Sea

In the not too distant past, war was viewed as an inherent right of the state, and states could resort to war for any reason at all, to include reprisal for perceived violations of international law,86 the precursor to reprisal actions as they are known today.87 This changed as a result of the horrors of two world wars during the first half of the twentieth century. By international agreement, the United Nations (U.N.) Charter foremost among them, and by the continuing evolution of CIL, there is now a general prohibition on the use of force between states.88 This general prohibition does not distinguish between conflicts on land and those at sea. As the San Remo Manual89 clearly states, “[t]he parties to an armed conflict at sea are bound by the principles and rules of international humanitarian law from the moment armed force is used.”90 Thus, the general prohibition against the use of force between and, more importantly, the exceptions to that general rule, apply at sea the same as they do on land and in the air.

A. Traditional Uses of Force—Belligerent Actions and Self-Defense

In keeping with the contemporary CIL prohibition against aggressive uses of force, the U.N. Charter and UNCLOS both require that states “refrain from any threat or use of force against the territorial integrity or political independence of any State, or in any manner inconsistent with the principles of international law embodied in the Charter of the United Nations.”91 The two clear exceptions to this prohibition on the use of force are belligerent uses of force authorized by the United Nations Security Council “to maintain or restore international peace and security,” and states exercising their inherent rights of self-defense.92 Both exceptions place the participants in a state of armed conflict, which is not addressed by UNCLOS, but is governed by CIL and various international agreements, such as Hague and Geneva Conventions.93 Nevertheless, the contemporary principles of self-defense and other relevant law of armed conflict provisions continue to apply equally at sea despite the omission from UNCLOS.94

Self-defense is, at its core, a high-level exercise in forcible self-help by a state.95 This application of lawful force as a remedy when the state’s rights have been violated by unlawful force “is and always has been one of the hallmarks of international law.96 Thus, even outside of a U.N. Security Council (UNSC) Resolution authorizing an offensive use of force, it is undisputed that a state may use the necessary97 and proportional98 force required to repel any attack against it at sea and to restore its national security.99 In fact, U.S. naval commanders are directly obligated to take necessary and proportional actions to safeguard their ship and crew.100 The extent to which defensive action may be taken “depends upon the intensity and scale of the armed attack . . . and the gravity of the threat posed.”101 While the right to use force in self-defense is clear, these references to “intensity,” “scale,” and “gravity” illustrate that CIL recognizes there are inherent differences in the degree of force and danger from which a state may have to take action to defend itself at sea.102

B. Other Permissible Uses of Force at Sea—Law Enforcement Activities

Additionally, there are other situations where a state may be permitted to use force against a foreign or state-less vessel, principally during law enforcement operations.103 “Navies and coast guards around the world may use force . . . if necessary for law enforcement in waters and for crimes over which they have enforcement jurisdiction.”104 Within internal and territorial waters, the ability to enforce domestic law is well-defined.105 Yet even outside of a state’s territorial seas, jurisdiction and authority may exist to use force to combat certain offenses by private entities and potentially even state actors, e.g., piracy and proliferation of weapons of mass destruction.106 In this context, the use of force in a law enforcement action is separate and distinguishable from the “armed force” referred to in the U.N. Charter;107 while it is to be avoided to the extent possible, it is permissible so long as it is “reasonable and necessary under the circumstances.”108 Specific international rules concerning the permissible uses of force in law enforcement actions are broad,109 but such uses of force include warning shots, disabling fire, stopping and boarding a ship, conducting searches, and arresting or seizing a ship and its crew.110 Although the legal basis for these actions stems from different authority, e.g., international laws against piracy,111 it is clear that there are incidents where force can be used outside the contexts of self-defense or armed conflict. Provided a given use of force is necessary and reasonable, i.e., proportional, “the ability to use force when needed to enforce international rights and compel compliance with lawful orders of a state is essential” to maintaining freedom of the seas.112

C. Countermeasures—More Than “Mere Pacific Reprisals”

In the same vein, legitimate self-help by a state to another state’s violations of international law may include countermeasures taken in reprisal.113

1. The Traditional View of Peaceful Countermeasures

Such self-help is a necessary component of international law because of the frequently criticized weakness of other international mechanisms for enforcement.114 For instance, repeatedly the UNSC has shown reluctance and incapability to take timely, definitive action in response to an act of aggression by one member state against another.115 In the absence or inability of international bodies to take action, individual states must be able to take action to compel compliance with international law.

Today, the terms “reprisal” and “belligerent reprisal” refer to actions taken in the course of an armed conflict, “countermeasures” are acts taken against a party outside of an armed conflict that would otherwise be unlawful in order to persuade that party to cease violating the law.116 In order to be justifiable, a countermeasure must meet certain conditions.117 In particular, such acts must rationally relate to the injury suffered, taking into account the gravity of the internationally wrongful act and the rights affected.118 “Judicial decisions, State practice and doctrine confirm the proposition that [non-forcible] countermeasures meeting certain substantive and procedural conditions may be legitimate.”119

2. The Need for, and Permissibility of, Forcible Countermeasures

But, can a state take forcible countermeasures in response to a use of force or other belligerent act that falls short of being a full, armed attack? If an “armed attack” means “a use of force causing human casualties or serious destruction of property,”120 then recent history is replete with examples of belligerent actions and uses of force falling short of an armed attack that nevertheless violated international law and the rights of other States.121 Many commentators argue, “[A]s a separate, although exceptional, means of the use of armed force, [self-help] has no support in contemporary international law.”122 Others argue that forcible countermeasures remain a part of CIL as a proportionate response to violations of international law not rising to the level of an armed attack for which traditional self-defense would be warranted.123 Even the International Court of Justice (ICJ), the judicial organ of the U.N., vaguely “hinted at the existence of a residual right of forcible reprisals outside the U.N. Charter” in its Corfu Channel case.124 While most academics and a cursory reading of the U.N. Charter support the former position, there are important exceptions that support the latter contention in favor of forcible countermeasures.

Notably, the 1986 ruling in the Case Concerning Military and Paramilitary Activities In and Against Nicaragua, the ICJ confirmed that although international law prohibits the use or threat of force between states that is supported by both treaty law (i.e., the U.N. Charter) and CIL, the general rule allows certain exceptions.125 In that case, Nicaragua brought proceedings in the ICJ against the United States126 for its support of and assistance to the military and paramilitary activities of the contra rebels in and against Nicaragua in the early 1980s, which included laying naval mines to block Nicaraguan harbors.127 The United States justified these actions as collective self-defense in response to Nicaragua’s prior comparable provision of arms and other logistical support to guerillas in El Salvador.128 While the ICJ ultimately found that the United States violated international law and the sovereignty of Nicaragua in several respects,129 the Court made two key observations throughout the course of its opinion regarding uses of force and countermeasures.

First, it distinguishes between different levels of force, separating “the most grave forms of the use of force (those constituting an armed attack) from other less grave forms” of force, aggression, and other acts for which self-defense under Article 51 of the U.N. Charter may not be appropriate.130 The Court found that where sending regular military forces or irregular armed groups across an international border to carry out acts of armed force would constitute an “armed attack” justifying acts of self-defense, other activities like the provision of weapons, supplies, and logistical or other support would not constitute an armed attack, even if such acts were still considered to be a threat or use of force.131 Under this reasoning, the United States was not entitled to exercise collective self-defense of El Salvador because Nicaragua’s support for the El Salvadorian guerillas did not constitute an “armed attack” that would trigger the right to self-defense.132 Consequently, the United States’ corresponding support for the contras, though not rising to the level of an armed attack under the same reasoning, violated the principles of non-intervention and non-use of force in international relations.133

Clearly, then, there exists a category of “less-grave” wrongful acts under international law that, despite violating international law, do not warrant acts of full, armed self-defense. This ludicrous discrepancy unacceptably leaves victim states in the vulnerable position of being unable to promptly respond to these wrongful acts, thereby encouraging further internationally wrongful acts by the offending state. If “violating the law of war, even in a manner it allows, is a repugnant act, yet an even more repugnant act is to allow an adversary to violate that same law with impunity.”134

Second, the Court implies that that a state that suffers a use of force not rising to the level of an armed attack for self-defense purposes may take proportional countermeasures.135 Specifically, the Court asked, and explicitly declined to definitively address, whether a state may “justify a use of force in reaction to measures which do not constitute an armed attack but may nevertheless involve a use of force.”136 However, the ICJ pointedly suggested that there might be “some right analogous to the right of . . . self-defence, one which might be resorted to in a case of intervention short of armed attack,”137 and “carefully refrained from ruling out the possibility that such counter-measures may involve the use of force by the victim State.”138

Essentially, the ICJ implies in the Nicaragua Case the existence of a right to employ legitimate countermeasures analogous to, but less serious than, a use of force in self-defense in response to a threat or use of force not constituting an armed attack, i.e., a use of force causing human casualties and/or serious destruction of property.139 In fact, the Nicaragua Case decision should be interpreted as “strongly suggest[ing] . . . that these counter-measures may include acts of force.”140

This view is not an aberration. Despite academic treatises and non-binding international resolutions to the contrary,141 state practice reaffirms the existence of countermeasures as a part of CIL.142 Moreover, the concept appears in other ICJ cases. In the Oil Platforms Case,143 the ICJ rejected Iranian claims that the United States breached a U.S.-Iran treaty by attacking an Iranian oil platform during the Tanker War144 and allegedly disrupting commerce between the two countries.145 However, the Court proceeded to unnecessarily analyze and determine that that U.S. attack did not constitute a valid exercise of self-defense.146 Prior to the attack, Iranian forces located on the oil platforms at issue launched attacks against U.S. flagged vessels in the area.147 Despite this provocation, the ICJ decided that the Iranian attacks did not constitute an “armed attack” justifying U.S. action in self-defense because the attacks were not of sufficient gravity to be an armed attack, and, allegedly, were not intentionally aimed at U.S. vessels.148 This matter was so contentious that five separate ICJ judges questioned the decision to address the issue,149 while one judge, Judge Simma expressly disagreed with the Court’s conclusion.150

Referring to precedent from the Nicaragua Case, Judge Simma believed that the ICJ cannot have meant “proportionate counter-measures” to mean “mere pacific reprisals” by the victim.151 He hints at the preposterousness of a victim of illegal actions not having “the right to resort to – strictly proportionate – defensive measures equally of a military nature.”152 Such a policy would further punish the victim of illegal actions, placing the onus on the victim state to accept current and potentially future instances of illegal actions by the offending state. Rather, Judge Simma “advocates a concept of defensive military action that falls short of ‘full-scale self-defence.’”153 He believes that CIL allows for a distinction for “hostile action, for instance against individual ships, below the level of Article 51, justifying proportionate defensive measures on the part of the victim, equally short of the quality and quantity of action in self-defence expressly reserved in the United Nations Charter.”154 Herein lies the crux of the argument for forcible countermeasures against, for instance, dangerous and unsafe activity by foreign or unknown drones.

V. Targeting Drones: Forcible Defensive Countermeasures

In the hypothetical scenario at the beginning of this article, an enemy drone interferes with aircraft carrier flight operations, impeding its mission and endangering the ship, its aircraft, and crew. Unfortunately, this is an all-too-real occurrence. On 8 August 2017, an Iranian Sadegh-1 UAV155 had engaged in an “unsafe and unprofessional” interaction with a U.S. F/A-18E Super Hornet.156 While the jet was preparing to land on the USS Nimitz, and despite repeated radio calls to stay clear of flight operations, the Iranian UAV passed within 100 feet of the F/A-18E, forcing the jet to take evasive maneuvers to avoid collision.157 Less than a week later, another Iranian UAV closed to within 1,000 feet of the USS Nimitz while it was engaged in night-time flight operations, and did so without utilizing standard, internationally-mandated navigation lights, which created a dangerous situation with a significant chance of collision.158 All told, in 2017, there were over fourteen dangerous incidents like this with Iranian drones.159 In each instance, not only did the Iranian drone violate international law by failing to act with due regard for navigational rights of the U.S. vessel, they created dangerous collision hazards that put U.S. personnel and property at risk.160 U.S. commanders must be able to respond, and forcible countermeasures may be an appropriate response.

A. A Lack of Due Regard that Endangers Human Life is Akin to a Use or Threat of Force Not Constituting an Armed Attack that Nevertheless Warrants Countermeasures

Intentionally or recklessly crossing the bow of a moving ship, failing to make way for larger ships and ships conducting flight operations, buzzing within 100 feet of other aircraft and ships, and similar maneuvers are hazardous actions that show a clear disregard to the danger of navigation and collision, and pose a danger to all craft and individuals involved.161 It does not matter whether their acts are committed intentionally, recklessly, or as a matter of simple negligence.162 Aside from violating principles of basic seamanship,163 such actions are also violations of international law. When state aircraft and vessels undertake dangerous maneuvers that fail to exercise due regard for the safety and navigational rights of other craft on, above, or below the water, it violates UNCLOS, the COLREGS, the Chicago Convention, and other international agreements.164 As discussed above, these laws apply equally to UAVs and UMS due to their respective statuses as State/Military Aircraft and vessels. Consequently, if a drone conducts dangerous operations like the ones described in the hypothetical and real-world examples above, this lack of due regard constitutes a violation of international law.

In considering the wide array of unlawful acts that are possible for one state to take against another, less-“grave” violations of maritime customs and laws, like the obligation of due regard, are unlikely to clearly constitute an armed attack by the state operating the drone. Nevertheless, these violations are acts akin to uses or threats of force that may warrant response under CIL as described by Judge Simma’s opinion.165 As noted above, the ICJ has held that otherwise non-violent provision of weapons, money, supplies, and logistical support to armed bands in another state, while not constituting an armed attack triggering the right of self-defense, may nevertheless be a “use of force of a lesser degree of gravity” that breaches the international prohibition against the use or threat of force between states.166 This is because actions like these, while not rising to a level that can be equated with an armed attack, still violate the general principle against the use of force, and are an unlawful intervention in the affairs of another state.167 Unsafe and unprofessional drone operations should be viewed in the same context; while not grave enough to be an “armed attack,” common sense and the evolving standards of warfare support the contention that such activity is akin to a use or threat of force because it interferes with the victim state’s affairs (i.e., serves as an impediment to the ship’s mission), jeopardizes its sovereign immune property, and endangers the lives of its personnel.

Consider the result of a pebble dropping into a pool of still water. The pebble does not merely break the surface and sink to the bottom without disturbing anything around it; the pebble creates a ripple on the surface of the water that spreads outward from the point of impact. Similarly, in real life, a drone does not pose a general danger that exists in a vacuum; it creates its own ripple effect on those vessels around it. When a drone fails to act with due regard for the safety and navigation of other vessels, its unsafe actions will force the victim vessels to take action to escape the threat. In the hypothetical, the presence of a foreign drone in the stack poses a danger to flight operations and the pilots involved, which may force the aircraft to cease operations or change course to steer away from the unsafe UAV.168 In the real-world interaction between the Iranian QOM-1 and the U.S. F/A-18E, the “unsafe and unprofessional” lack of due regard exhibited by the Iranian drone forced the U.S. F/A-18E to take evasive action in order to avoid collision.169 Most importantly, the dangerous drone activity is not a threat in and of itself, or simply because it threatens potentially extensive damage to other vessels and craft; it also poses a very real threat to the lives of the sailors and aviators involved. Accordingly, the dangerous lack of due regard by a drone that violates international law, impedes operations, and poses a danger to human life is analogous to a traditional use or threat of force.

B. Use of Force Against an Unmanned Drone is a Proportionate Countermeasure

Since dangerous drone activity in violation of international law is analogous to a use or threat of force short of an armed attack, the victim state must be able to employ proportionate, forcible countermeasures to safeguard its personnel and to deter future violations. To outright prohibit forcible countermeasures in all situations, as some in the international community seek to do, is an unnecessary restriction that controverts CIL170 and “seriously thwarts” the “right of [a] victimized state to secure respect for its legal rights and interests. . . .”171 Thus, prohibition creates an absurd catch-22;172 limiting the use of forcible countermeasures places the onus on the victim state to either accept repeated violations of its rights and international law, thereby encouraging further violations and allowing the danger to its property and personnel to continue, or to commit its own violation of international law as it seeks to take defensive action.

Recall that countermeasures are acts taken against a party outside of an armed conflict that would otherwise be unlawful in order to persuade that party to cease violating the law.173 The hypothetical and real-world examples above both involve an unmanned vehicle, outside of an armed conflict, acting in a manner that violates international law and poses a danger to personnel. Moreover, these violations of state rights and international law are a frequent occurrence by certain states.174 In such a situation, the United States should be entitled to employ proportionate forcible countermeasures short of full self-defense to defend its property and personnel, and to dissuade the offending state from repeating such actions in the future. As forcible countermeasures are “analogous to the right of . . . self-defence,”175 then the proposed countermeasures must comply with the same requirements of necessity and proportionality for self-defense actions.176 If a forcible countermeasure complies with these requirements, then a victim state would be acting within the ambit of traditional state practice, CIL, and ICJ judgments.

Necessity for forcible countermeasures is established in two ways. First, the victim state has an interest in taking action to counter the offending state’s violations of international law and to deter future violations.177 In an era of warfare that will increasingly rely on drones,178 the knowledge that violations of international law by drones will be met with appropriate, forcible countermeasures will discourage such actions. Second, the victim state obviously has a significant interest in taking on-the-spot action to end the threat to its property and personnel, which, as noted by Judge Simma, may consist of defensive measures of a military nature.179

Proportionality is a higher bar to meet. The justification for forcible countermeasures “analogous to but short of self-defence” is another state’s prior use or threat of force “less grave than an armed attack,” i.e., “a use of force causing human casualties and/or serious destruction of property.”180 Just like traditional self-defense, proportionality does not limit the countermeasure to the exact manner and level of force used in the initial unlawful action by the offending state. Rather, proportionality permits the level of force needed to respond to, and effectively deal with, the threat or danger, which can be greater than the initial threat so long as the result is still roughly proportionate.181 Hence, in response to a use or threat of force below the level of an armed attack, it would be grossly disproportionate to employ a forcible countermeasure so severe that it constitutes an armed attack itself. For instance, the hypothetical would likely fail the test of proportionality to launch a missile strike against the enemy drone’s operators, home base, or mother ship, since the countermeasure would be a far more severe and direct use of force.

Targeting the drone itself, however, would be a proportional countermeasure. Use of force against the drone is directly related and responsive to the drone’s unlawful actions. The result achieved, elimination of the drone and the threat it poses, is a relatively symmetric and balanced result compared to the dangerous, unlawful actions of the drone. Assuming the incident does not occur while exercising innocent passage through the territorial seas of the state operating the drone, then there is no violation of the territorial sovereignty or political independence of the offending state, further precluding any violation of the U.N. prohibition on the use of force.182 Most importantly, by destroying a single, unmanned vehicle, the countermeasure avoids any civilian casualties or more significant destruction of property.183 Consequently, the countermeasure is not “grave” enough to constitute an armed attack, and is arguably a lesser use of force compared to the threat to human life posed by a drone acting without due regard for safety and navigation rights. In this manner, the same unmanned nature of drones that makes states more likely to utilize them also weighs in favor of them being the lawful target for forcible countermeasures.

Additionally, if destruction of the enemy drone is a proportionate response, then lesser, non-kinetic countermeasures such as electronic warfare (EW) measures to disable or drive off the drone would also be permissible since they achieve the same desired result—elimination of the threat posed by the drone. This is in keeping with principles of effects-based targeting, where all possible means drawing from any available forces weapons and platforms are considered to achieve specific, desired effects.184 If lesser, non-kinetic countermeasures would achieve the same result, i.e., end the dangerous actions of a drone, and time and circumstances permit, then such measures should be considered in keeping with traditional notions on escalation of force.185 Similarly, if time and circumstances permit, commanders may attempt to de-escalate an encounter with an unprofessional and unsafe drone without use of countermeasures,186 e.g., by changing course. This may especially be the case depending on the legal status of the seas the action takes place in.187 However, this will be highly dependent on the specific circumstances, and commanders are not required to de-escalate a situation or attempt to use non-destructive measures as a precursor to use of more powerful measures.188

Additionally, EW measures may not be as quick or decisive in eliminating a drone threat as a traditional kinetic countermeasure that destroys the target drone. Also, EW measures may not eliminate the threat if such measures, even if disrupting its communications link with its controllers, leave the drone in an area where its mere presence poses a continuing danger to navigation, e.g., the stack in the hypothetical. Accordingly, where circumstances permit, a commander should consider non-kinetic measures against an illegal drone, but the commander is not required to exhaust such measures before resorting to a kinetic strike.

Thus, forcible, destructive action against an enemy drone that violates international law by failing to act with due regard for other aircraft and vessels, thereby endangering those other craft and their human crews, is a proportionate forcible countermeasure.

C. A Framework for Forcible Countermeasures

As unreasonable as it would be to completely prohibit forcible countermeasures falling short of actions taken in self-defense, as shown above, it would be inappropriate and intellectually dishonest to argue that forcible countermeasures, even if proportionate and necessary, are appropriate in all circumstances. Like any use of force, the specific facts of each situation and many other factors must be considered prior to authorizing the action.189 In order to be feasible and defensible in real-world drone encounters, a framework is needed to delineate relative levels of forcible countermeasures that may be employed depending on the nature of threat, the area of the seas where the encounter occurs, and the degree of damage that the countermeasure will cause.

1. Dangerous Violation of International Law

As discussed above, forcible countermeasures will not be permissible in all circumstances.190 By their very nature, countermeasures, just like belligerent reprisals, are acts that would be unlawful but for their use in response to another state’s prior violation of international law, and that are taken for the purpose of deterring future violations of international law.191 Consequently, any use of countermeasures must be in response to a violation of international law.

Moreover, to specifically warrant a forcible countermeasure, regardless of the level of force to be used, that violation of international law must pose a life-threatening hazard to the safety of U.S. vessels or aircraft, and the lives of embarked Sailors and Airmen.192 Mission impediment, e.g., obstructing intelligence collection or interfering with a freedom of navigation operation, is not enough. It is the grave danger to U.S. forces that is analogous to a use or threat of force short of an armed attack, which is what triggers a potential forcible countermeasure as a response short of self-defense.193

Therefore, to justify use of a forcible countermeasure, an offending state must commit a violation of international law that poses a life-threatening danger to U.S. forces.

2. Different Maritime Regimes Under the Law of the Sea

A foundational premise of the law of the sea is that the sovereignty of coastal states extends, under different legal classifications, beyond its land territory and internal waters.194 Even if U.S. commanders encounter a life-threatening breach of international law that potentially warrants a forcible countermeasure, the location where the breach occurs will have a significant impact on whether forcible countermeasures may be used and the level of force involved. This is because the seas are nominally reserved for peaceful purposes, and states are obligated to avoid threats or uses of force against the sovereignty, territorial integrity, or political independence of other states.195 And as shown in the following chart, the closer a foreign state’s vessel is to a coastal state, the greater the degree of control that the coastal state can generally exert over the surrounding area and the conduct of foreign craft therein,196 and the greater the limitations on the foreign state vessel while transiting the coastal state’s waters.197

198

a. The High Seas and EEZs – Full Freedom of Navigation

Throughout the high seas,199 commonly referred to as international waters, and any coastal states’ EEZs,200 the full panoply of navigational rights are available to ships of all nationalities. This is known as the freedom of the high seas,201 and it is part of what makes the world’s oceans and the resources contained therein the “common heritage of mankind.”202 Even within an EEZ, although the cognizant coastal state retains sovereign rights of the resources within the EEZ, all other high seas freedoms, such as navigation and overflight, are available to foreign states. Although the high seas and EEZs are nominally “reserved for peaceful purposes,”203 which some states conservatively interpret by making excessive maritime claims that purport to regulate military activity of foreign states within such areas ,204 it is broadly agreed upon that all states may conduct military operations in EEZs and on the high seas,205 subject only to the requirement that they exercise due regard for the interests of other states.206 Accordingly, a coastal state has little to no control over foreign military naval activity occurring on the high seas or an EEZ.207

b. Territorial Seas – Innocent Passage

A coastal state may exert significant control over its territorial seas, which is an area of sovereign territory extending up to a maximum of 12 NM from the coast.208 Within a coastal state’s territorial sea, foreign vessels are subject to the coastal states laws, which gives the coastal state a vested interest in regulating the conduct of all ships therein.209 Nevertheless, international law requires that coastal states “shall not hamper” foreign ships’ passage through their territorial seas,210 and particularly proscribes taking any action or impose any requirement that has the “practical effect of denying or impairing” the right of passage.211 Under this regime, foreign states may traverse a coastal state’s territorial sea by exercising the right of innocent passage,212 or the right of transit passage through international straits.213

Both innocent passage and transit passage must be conducted continuously and expeditiously, and must refrain from “any threat or use of force against the sovereignty, territorial integrity or political independence of the coastal State.”214 Innocent passage is particularly restrictive, and requires that foreign ships refrain a number of activities, including any exercise or practice with weapons of any kind, and the launching, landing, or taking on board of any aircraft or military device.215 These restrictions are based on large part of the recognition that the territorial sea is the sovereign territory of the coastal state. A threat or use of force, or other provocative activity like those listed in Article 19,216 may be deemed to be a threat to the territorial integrity and political independence of the coastal state itself.

c. International Straits and Designated Archipelagic Sea Lanes—Transit Passage

The independent legal regime of transit passage is different, and it occupies a place in between freedom of navigation and innocent passage.217 Though it also involves traversing a coastal state’s territorial seas in a continuous and expeditious manner,218 the limitations on transit are not as restrictive. A foreign state’s ship must still avoid “any threat or use of force against the sovereignty, territorial integrity or political independence of States bordering the strait,” but ships exercising transit passage through international straits are allowed to engage in activities that are “incident to their normal modes of continuous and expeditious transit.”219 The United States interprets this provision as allowing submarines to remain submerged during transit, and for aircraft carriers to conduct normal flight operations during the same.220 In this fashion, although coastal states retain their normal sovereignty rights over the territorial seas comprising international straits,221 international law acknowledges that there is a significant difference between transit through an international strait and mere innocent passage through a coastal state’s territorial sea, and provides ships transiting international straits greater rights.222

3. Levels of Force and Destructiveness

Technologically advanced forces like the U.S. military have multiple responsive measures that may be used against an offending state’s drone.223 Electronic warfare measures can jam a drone’s communications link with its operator, making the drone inoperable, and potentially making it automatically return to its home base.224 It is even possible for EW measures to cause the destruction of a drone by accessing and disrupting internal processes.225 Finally, a ship can take direct action to target and destroy an offending state’s drone using traditional kinetic weapons, or even directed energy weapons.226 The U.S. Navy and other military entities are continuously seeking out new and better measures to counter enemy drones.227 Yet the sophistication of a particular weapon or EW measure is not dispositive of its use as a countermeasure; the likely result of its utilization, i.e., how forceful or destructive it will be, is the measure that will impact its use. For instance, there may be situations where a forcible countermeasure is warranted, but where it would be a more appropriate level of force to employ EW measures to disrupt a drone’s signal as opposed to obliterating the drone with a surface-to-air missile, thereby avoiding the destruction of another state’s property and follow-on risk of escalation.

4. Implementing a U.S. Countermeasure Framework

By considering the nature of the threat posed by a dangerous drone encounter, balancing the rights and obligations of coastal states and foreign states based on the particular law of the sea regime, and contemplating the types of potential countermeasures and their relative levels of destructiveness, a naval commander can ascertain parameters for possible forcible countermeasures as detailed in the following chart and below text:

a. No Countermeasures While Exercising Innocent Passage

Within the normal territorial sea of a coastal state, whether by invitation of the coastal state or through the exercise of innocent passage, forcible countermeasures against a drone or other vessel belonging to the coastal state are inadvisable as a matter of prudence. Although the obligation of due regard is a universal requirement that applies in all maritime areas,228 and a drone’s failure to act with due regard is a violation of international law giving rise to the legal right to use forcible countermeasures, certain factors vitiate use of forcible countermeasures in the coastal state’s territorial sea. First, a territorial sea is the sovereign territory of the coastal state;229 any forcible countermeasure employed in the coastal state’s territorial sea could be viewed as a violation of the territorial integrity and political independence of the coastal state itself, which would be a violation of international law,230 and possibly an illegal armed attack triggering the coastal state’s right of self-defense.231 Second, the coastal state’s rights to regulate conduct of ships at sea is greatest in this area, and the activities of foreign ships traversing the territorial sea by exercising innocent passage are heavily restricted.232 This limited nature of operations that U.S. ships may conduct during innocent passage, e.g., no flight operations for an enemy drone to disrupt, make the need to use countermeasures less likely. Third, should forcible countermeasures be employed, this would violate our own obligation to refrain from any use of weapons or other acts “prejudicial to the peace, good order, and security of the coastal State.”233 Consequently, our ships’ passage would no longer be innocent, entitling the coastal state to eject the ship from the territorial sea.234

However, this restriction on use of countermeasures in the territorial sea should not apply to third party drones operating in the territorial sea. In such a situation, employment of a forcible countermeasure against a third-party state to whom the drone belongs, rather than the coastal state itself, would obviate the aforementioned concerns. Additionally, a restriction on the use of countermeasures in the territorial would also not preclude actions taken in self-defense if the coastal state commits a use or threat of force that constitutes a hostile act or demonstrated hostile intent.235

b. Non-Destructive Countermeasures in International Straits

Within international straits and archipelagic sea lanes, although the waters themselves are technically still part of the coastal state’s territorial seas,236 these waterway are addressed by separate legal regimes that provide greater rights to foreign ships transiting the area.237 Moreover, coastal states are prohibited from suspending transit and must refrain from acts that have the practical impact of denying transit passage.238 In this manner, UNCLOS treats international straits and archipelagic sea lanes more like the high seas than as part of the territorial sea. Consequently, foreign ships exercising transit passage may be permitted a greater degree of flexibility in responding to a dangerous violation of international law by a coastal state’s drone.

To a degree, the concerns weighing against use of countermeasures in the territorial sea still apply. However, non-destructive forcible countermeasures that eliminate the threat posed by the drone without destroying the drone itself, such as use of EW to disrupt a drone’s communications signal, would be appropriate responses to dangerous drone encounters in international and archipelagic straits. More importantly, use of non-destructive countermeasures avoids any potential violation of the territorial integrity or political independence of the coastal state. Such measures appropriately balance the naval commander’s need to counter the direct threat posed by the drone and deter future violations of international law, with the due regard required for the coastal state’s continuing, albeit reduced, right under international law to regulate that portion of its sovereign territory. As noted above, this would neither preclude the ability to use stronger, more-destructive forcible countermeasures against a third-party drone operating without due regard in international straits, nor would it impact the right to use force in self-defense against a hostile act or demonstrated hostile intent by the coastal state.

c. Full Spectrum of Countermeasures on the High Seas

Finally, full forcible countermeasures up to, and including, destruction of a dangerous drone are permissible on the high seas and in EEZs. No state can regulate or impede the rights of navigation and overflight in these areas;239 any action by a state to do so through dangerous, harassing drone activity that violates international law can and should be met with full forcible countermeasures. There is also no requirement under the U.N. Charter, UNCLOS, or CIL requiring that lesser, non-destructive countermeasures be attempted first.240 Lesser measures may not be successful, and even if successful, may leave an uncontrolled drone in a position that still poses a danger to safety and navigation. Absent the need to consider the territorial integrity or political independence of a coastal state, full forcible countermeasures that result in destruction of the offending drone are permissible.

VI. Conclusion

Despite the general international law disapprobation for armed countermeasures and the use or threat of force between states, forcible countermeasures are an important and necessary right under CIL. States and their naval commanders should, and are legally entitled to, employ appropriate forcible countermeasures analogous to, but falling short, of use of force in self-defense against dangerous violations of international law not acts not rising to the level of an armed attack. This is particularly important in the modern operating environments, where unmanned vehicles frequently violate international law with impunity and pose an increasing threat to the safety and navigation of naval forces. The United States should adopt and implement a countermeasure framework as described above to combat these dangerous violations by drones by utilizing proportionate defensive countermeasures up to, and including, blowing the drone out of the sky or water.