Vessels of War

How can vessels be used in war? Are rights and obligations vested in armed forces personnel or in the naval platforms themselves? Combatancy in land warfare is fairly straightforward. Persons sanctioned by and under the control of a State have obligations under the law of war and, in turn, may use force and exercise other belligerent rights on the State’s behalf. In naval warfare, the answer is more complicated.

The nature of the maritime domain, with its unique balance of neutral and belligerent rights, has led to various piecemeal agreements and proposals for additional State control since the 19th century. One byproduct of this process has been the now-common assertion that only vessels satisfying certain criteria can be used to exercise belligerent rights at sea during an international armed conflict. However, this association of a “warship definition” with belligerent rights is a concept in need of reexamination in light of modern conditions and the expected nature of future naval engagements.

The “Warship Definition”

At its origin, the “warship definition” encapsulated the ways a State could fulfill its obligations while exercising belligerent rights at sea. However, this never constituted a separate legal requirement. The customary law of naval warfare requires States to ensure that belligerent rights at sea are exercised by lawful combatants and in accordance with the principles of the law of war. With the development of modern technology, training, and tactics, which have driven the steady evolution of practice over the last 70 years, States have other options for fulfilling their legal obligations. As a result, so long as States comply with these obligations, they can lawfully conduct belligerent activities from a naval auxiliary or any other platform at sea.

To understand the need for an updated understanding, it is significant to examine how classification as a “warship” came to be associated with belligerent rights and, more importantly, what legal concerns, rights, and responsibilities of States it encompassed in its time.

The criteria of the “warship definition” came together from several connected trends between 1700 and 1900. As others have identified, the most important were the international push to increase State control of violence at sea and end privateering, the need to distinguish merchants armed for defense from vessels used for combat, and the maturation of neutral rights at sea (p. 463-65). Rather than a binding definition, the criteria provided a shorthand for ways a State could promote these objectives and originated mainly to constrain the assertion of “belligerent rights” over neutral commercial vessels (p. 228-30).

Despite these origins, the “warship definition” is often understood overly restrictively as an additional constraint on State execution of belligerent acts at sea against enemy military objectives. There is no grounding in treaty or clear custom for such a requirement; the closest approach is the Hague Convention (VII) Relative to the Conversion of Merchant Ships into War Ships (Hague VII). From its negotiating history, this treaty clearly aimed to prevent a return of privateering in another form (p. 744-1086).

Instead, the jus in bello obligations of States executing belligerent acts at sea are the same as those on land (§ 5.1.2). States must ensure that lawful combatants exercise belligerent rights on their behalf, and that combatants use offensive force only as necessary, with distinction, in accordance with proportionality, without causing unnecessary suffering, and within the bounds of military honor, particularly without resort to perfidy.

The relevant “warship definition” criteria—external markings distinguishing the character and nationality of such ship, serving under the command of a duly commissioned officer, and being manned by a crew that is under regular armed forces discipline—were one way of satisfying these requirements, focused particularly on protecting neutral rights. The requirement for external markings provided neutrals some assurance of the State sanction and legitimacy of the vessel and crew, when ordered to stop for visit and search. Command by a duly commissioned officer similarly ensured State authority. In addition, the requirement of a crew under regular armed forces discipline provided a clear line of State control and responsibility for the actions taken from a vessel. Both were intended to ameliorate the worst abuses of prize law, and reinforce the protection of neutral commerce.

Contemporary Considerations

To paraphrase the U.S. Department of Defense Law of War Manual, persons, not inanimate objects such as weapon systems, have rights and obligations under the law of war (§ 6.5.9.3). Vessels designed to attack other vessels are not actors with rights and obligations under the law. They are instead a means of war; they are “weapons or devices used to conduct warfare.” Requiring such vessels to have particular markings during attack supports the general requirement of affirmative distinction. Also, requiring control of force from such vessels by State armed forces ensures attacks are conducted by lawful combatants. As the conduct of war evolves, however, particularly with the continued development of cyber, space, and other information operations, new forms of belligerent acts develop.

This is not the first such situation the law of naval warfare has faced, merely the latest. The submarine presented a similar, seemingly novel challenge (p. 453, 459, 478). After long debate and much wringing of hands, nations settled on requiring compliance by the persons controlling such vessels with the customary law of war principles, rights, and obligations. Execution of current belligerent activities may be “from” vessels, but only insofar as the vessel provides geographic basing or a transmission point. Examples are the U.S. Navy’s cooperative engagement and fire-on-remote capabilities, allowing coordinated, correlated employment of sensors and weapon systems across dozens of platforms and hundreds of miles. In many cases, these belligerent activities are “conducted,” in the sense of command and control, by persons physically located elsewhere. Such acts should not be limited to “warships” when they are otherwise legal.

For example, if a State conducts a cyberattack from a computer on board a vessel or transmits via a radio signal from such a vessel, the State’s law of war compliance is unaffected by factors such as whether the master of the vessel is commissioned or the crew is subject to armed forces discipline. These individuals are not controlling, conducting, or executing the belligerent act. Instead, as Professor Harold Koh and Brian Egan have addressed, it is important that the decision to conduct the cyberattack is made by an officer sanctioned by a State and that this decision is carried out by personnel responsible to that State under armed forces discipline. Those personnel have been trained and will be accountable for exercising distinction, weighing proportionality, and preventing unnecessary suffering. The concerns for neutral rights from the late 19th century, and the anti-privateering aims of the Paris Declaration, Hague VII, and the Oxford Manual, are not at play in this scenario. Despite a civilian mariner acting as the ship’s master, and the crew being comprised of civilian mariners, the cyber activity would be conducted by armed forces personnel who are trained and accountable to the State under the law of war.

The requirements of affirmative distinction still apply. All States have an interest in not increasing the risk of mistaken targeting or collateral effects to civilians or neutrals beyond the bounds of military necessity. The practice of maritime fires, however, accounts for the nature of the operational environment and of naval weapons, allowing for broad ruses of war and exposure of a vessel’s military nature only at the moment of attack. While the United States has often separated “warships” from “auxiliaries” based on their design and physical capabilities, both are also clearly marked as vessels in service to its military. Delegates negotiating the UN Convention on the Law of the Sea also recognized the broader types of vessels used by States in the modern period to execute traditionally naval activities, by allowing such activities from any other ship duly authorized or identifiable as on government service (see e.g. arts. 107, 110, 111, 224). With combatants executing actions under the law of war, the cyber action described above is permissible from an auxiliary or other government vessel so long as they comply with the rules against perfidy or other impermissible ruse.

Kinetic effects at sea should be assessed using the same framework. To paraphrase Brian Egan, the same rights and obligations apply to both traditional and cyber military operations during armed conflict. If a containerized munition was embarked on the deck of a naval auxiliary, the master of that vessel would not need to be a commissioned officer, nor the crew under armed forces discipline, to ensure in bello requirements are met. Those persons are engaged in the transport of a munition, an action which is not itself an attack or part of an attack. The movement of weapons into a conflict zone over the sea, to where they will be militarily useful, has regularly been undertaken by civilians employed by the government; by granting them prisoner of war status when captured, the Geneva Conventions recognize the particular status of these mariners and their lawful contributory role to warfighting (art. 13(5)). As with the cyber activity discussed above, so long as the launch and control of the munitions against the enemy is under the command and control of lawful combatants, the State would satisfy its in bello obligations, even if the missile issues from an auxiliary. Concerns about neutral rights and affirmative distinction would also be met by ensuring the vessel is properly identified as acting on behalf of the belligerent State.

Concluding Thoughts

The “warship definition” was never intended to serve as a separate legal requirement. Instead, it was a convenient shorthand for ways that different naval warfare obligations could be met with the technology of the time, focused on particular concerns of that era. Many of those same concerns remain, particularly with respect to the protection of civilians during conflict. However, the technology, training, and tactics of modern naval warfare have provided other, better ways to address them.

The practice of maritime nations over the last 70 years has shown our ability to appropriately use civilians in combat support roles, to command and control uses of force from a distance, and to protect neutrals and civilians from abuse by focusing on the principles of the law of war instead of outdated and generalized formulas. The “warship definition” reflects the concerns and solutions of the late 19th and early 20th century, not the present day. Limiting the conduct of belligerent acts to vessels satisfying the warship criteria serves as a distraction from actual jus in bello compliance and results in an unwarranted tax on military effectiveness. The actual legal obligation of States during naval warfare is compliance with the customary law of war principles. Satisfying the traditional “warship definition” criteria is no longer an effective way of doing so.

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Commander Lena E. Whitehead is currently serving as the Force/Fleet Judge Advocate for COMNAVCENT/C5F.

Commander Aaron Waldo is currently assigned as Sensitive Programs Counsel at OJAG, Code 10.

Photo credit: U.S. Navy, Seaman Geoffrey Ottinger