The conduct of hostilities at sea, commonly known as naval warfare, is governed by a complex and multifaceted body of international law designed to regulate the use of force and mitigate human suffering. These rules form a specialized branch of public international law, drawing primarily from international humanitarian law (IHL), also known as the law of armed conflict (LOAC), and public international law of the sea. The overarching objective is to strike a delicate balance between military necessity and humanitarian considerations, ensuring that even in the brutal environment of armed conflict, fundamental principles of humanity and proportionality are upheld.

The evolution of these rules has been incremental, shaped by centuries of maritime practice, customary law, and a series of international treaties and conventions. From early prize law and rules of engagement concerning merchant vessels to the modern framework encompassing submarines, aircraft, and sophisticated naval technology, the legal regime has adapted to changing realities while retaining its core humanitarian purpose. Key instruments like the Hague Conventions (particularly Hague VIII concerning automatic submarine contact mines), the Geneva Conventions (especially Geneva Convention II on Wounded, Sick and Shipwrecked Members of Armed Forces at Sea), and the United Nations Convention on the Law of the Sea (UNCLOS) provide the foundational framework, further elaborated by influential manuals like the San Remo Manual on International Law Applicable to Armed Conflicts at Sea.

Sources and Foundational Principles

The rules governing naval warfare derive from several authoritative sources of international law. Primary among these are customary international law, which evolves from the general and consistent practice of states followed by them from a sense of legal obligation (opinio juris), and treaty law, comprising ratified international agreements. Significant treaties include the aforementioned Hague Conventions, particularly those regulating the means and methods of warfare, and the four Geneva Conventions of 1949 and their Additional Protocols, which primarily focus on the protection of victims of armed conflict. The UNCLOS, while primarily a law of peace, is crucial for defining maritime zones and the rights of navigation, which become critical considerations during armed conflict. Additionally, non-binding but highly influential documents, such as the San Remo Manual on International Law Applicable to Armed Conflicts at Sea (1994), provide detailed interpretations and elaborations of existing law, serving as valuable guidance for naval commanders and legal advisors.

Underpinning all aspects of naval warfare are the fundamental principles of international humanitarian law. The principle of distinction mandates that parties to a conflict must distinguish between combatants and civilians, and between military objectives and civilian objects. Attacks must only be directed against military objectives. At sea, this means distinguishing between warships and other legitimate military vessels or facilities, and civilian merchant ships, fishing vessels, or passenger liners. The principle of proportionality prohibits attacks that may be expected to cause incidental loss of civilian life, injury to civilians, damage to civilian objects, or a combination thereof, which would be excessive in relation to the concrete and direct military advantage anticipated. This requires commanders to weigh the military gain against potential civilian harm. The principle of military necessity permits measures which are indispensable for securing the complete submission of the enemy as soon as possible, and which are not forbidden by international law. However, this necessity is not absolute; it must always operate within the bounds set by the principle of humanity. The principle of humanity prohibits the use of weapons, methods, or means of warfare that are of a nature to cause superfluous injury or unnecessary suffering. It also dictates that persons rendered hors de combat, such as the wounded, sick, or shipwrecked, and prisoners of war, must be treated humanely.

Combatants and Protected Persons at Sea

In naval warfare, combatants are members of the armed forces of a party to an armed conflict. This includes naval personnel and marines, who are legitimate targets of attack unless they are hors de combat. Prisoners of War (POWs), once captured, are entitled to the protections afforded by Geneva Convention III, including humane treatment, proper food, medical care, and protection from violence or reprisal. They must be released and repatriated without delay after the cessation of active hostilities.

Wounded, Sick, and Shipwrecked (WSS) personnel at sea are protected by Geneva Convention II. This convention provides for their humane treatment, medical care, and protection from further harm. Hospital ships, clearly marked and operating in accordance with the Convention, are granted absolute protection from attack. They may not be used for any military purpose and must carry no weapons other than those necessary for their defense or maintaining order on board. Medical aircraft operating over the sea are similarly protected. Medical personnel aboard such vessels are also protected and must be allowed to carry out their duties. Any use of a hospital ship for military purposes, or its failure to adhere to the marking requirements, can lead to a loss of its protected status, though even then, a warning must be given and a reasonable time allowed for the infringement to cease before an attack.

Civilians at sea, including passengers and crews of merchant vessels (unless these vessels are participating directly in hostilities), are protected persons. They must not be made the object of attack. Direct attacks on passenger ships, fishing vessels, or other purely civilian craft are strictly prohibited unless they lose their protected status by directly participating in hostilities.

Military Objectives and Prohibited Means of Warfare

A military objective at sea is defined as an object which by its nature, location, purpose, or use makes an effective contribution to military action and whose total or partial destruction, capture, or neutralization, in the circumstances ruling at the time, offers a definite military advantage. This encompasses warships, military aircraft, naval bases, port facilities used for military purposes, and auxiliary vessels supporting military operations. Certain merchant vessels may also become legitimate military objectives if they are integrated into the enemy’s war-fighting machinery (e.g., troop transports, dedicated supply ships for military operations, vessels laying mines, or those resisting visit and search by belligerent warships).

The law of naval warfare strictly prohibits certain methods and means of warfare. Indiscriminate attacks that are not directed at a specific military objective, or which employ a method or means of combat that cannot be directed at a specific military objective, or whose effects cannot be limited as required by IHL, are forbidden. This includes blanket bombardments of sea areas or ports without specific military targets. The use of weapons causing superfluous injury or unnecessary suffering is also prohibited. While there is no specific treaty prohibiting certain conventional naval weapons globally, the general principle of humanity applies, meaning that weapons whose sole purpose is to inflict particularly cruel injury are banned. The deliberate or widespread destruction of the natural environment as a weapon of war is also proscribed, particularly if it is disproportionate to the military advantage gained. Perfidy, which involves feigning protected status (e.g., flying a neutral flag to gain a military advantage before an attack, or feigning distress), is strictly prohibited as it undermines the very protective regime of IHL. However, ruses of war, such as disguising a warship as a merchant vessel or flying a false flag before engaging an enemy ship (provided the correct flag is hoisted before opening fire), are generally permissible, as they do not involve feigning protected status under IHL.

Rules for Specific Vessels

The legal status and treatment of vessels vary significantly depending on their nature and activities. Warships are vessels belonging to the armed forces of a state, bearing the external marks distinguishing vessels of their nationality, under the command of an officer duly commissioned by the government of that state, and manned by a crew which is under regular armed forces discipline. They possess sovereign immunity on the high seas and in the EEZ, and their actions are regulated by the rules of naval warfare. They have the right of visit and search, and can capture enemy vessels.

Merchant vessels are generally civilian objects and presumed to be protected from attack. However, their protection is conditional. A merchant vessel loses its protection and becomes a legitimate military objective if it makes an “effective contribution to military action” for the enemy. Examples include transporting troops or military cargo, acting as a scout or picket for the enemy navy, laying mines, or resisting lawful visit and search or capture. If a merchant vessel resists capture, it may be attacked. If it attempts to escape after being ordered to stop, it may also be pursued and forced to stop, potentially through force. The destruction of a captured enemy merchant vessel is permissible only under strict conditions: if the vessel cannot be diverted to a friendly port, if its destruction is necessary for military operations, and if the safety of the crew and passengers can be ensured (e.g., by taking them aboard the capturing warship and ensuring their safety before sinking the vessel). This rule, often referred to as the “cruiser rules” or “prize rules,” was historically challenged by unrestricted submarine warfare, which often involved sinking merchant ships without warning or provision for the safety of the crew. Modern law re-emphasizes the need to prioritize the safety of non-combatants.

Hospital ships, as discussed, enjoy special protection under Geneva Convention II. They must be clearly marked with the red cross or red crescent on a white background, and cannot be used for any military purpose. They are immune from capture and attack, even if carrying sick or wounded enemy personnel.

Neutral vessels belonging to a neutral state are generally immune from belligerent attack. However, they are subject to certain obligations and limitations. They cannot participate directly in hostilities, nor can they carry contraband (goods destined for the enemy that are useful for its war effort) or breach a lawful blockade. If a neutral vessel is found to be carrying absolute contraband (items exclusively military in nature, like weapons) or conditional contraband (items with dual civilian and military use, like fuel, if destined for the enemy’s armed forces or military installations), or attempting to breach a blockade, it may be stopped, visited, searched, and potentially captured and brought before a prize court.

Fishing vessels and small coastal craft engaged in purely civilian activities, like local fishing or transport, are generally protected. They lose this protection if they are incorporated into the enemy’s war effort or used for military purposes.

Blockade, Contraband, and Prize Law

Blockade is a legitimate method of naval warfare used to prevent ingress and egress of vessels and aircraft from enemy ports, coastlines, or other specified areas. For a blockade to be lawful, it must meet several conditions:

  1. Notification: It must be declared and duly notified to neutral states, specifying the commencement, duration, geographical limits, and the period within which neutral vessels may leave the blockaded area.
  2. Effectiveness: It must be effective, meaning it is maintained by a force sufficient to prevent passage, or at least to create an evident danger to ships attempting to enter or leave the blockaded area. A mere declaration without effective enforcement is not enough.
  3. Impartiality: It must be applied impartially to vessels of all nations.
  4. No disproportionate harm to the civilian population: While the primary purpose is military, it should not starve the civilian population or deprive it of essential foodstuffs or medical supplies if these are the only means of entry. Humanitarian relief efforts generally have a right of passage, subject to inspection.

A vessel attempting to breach a lawful blockade, whether enemy or neutral, may be captured.

Contraband refers to goods which are liable to capture if they are being transported by a neutral vessel to an enemy destination during armed conflict. Contraband is divided into:

  1. Absolute contraband: Goods exclusively useful for war (e.g., arms, ammunition, military equipment). These are always liable to capture if destined for enemy territory.
  2. Conditional contraband: Goods that can be used for both peaceful and military purposes (e.g., food, fuel, raw materials). These are liable to capture only if it is proven they are destined for the enemy’s armed forces or military administration. The destination must be established through evidence, such as ship’s papers or other intelligence.

A neutral vessel found carrying contraband may be subject to visit, search, and potentially capture. The contraband goods may be seized, and under certain circumstances, the vessel itself may be condemned as a prize.

Prize law is the body of law that governs the capture of enemy or neutral property at sea during armed conflict. When a belligerent warship captures an enemy vessel or neutral vessel carrying contraband or breaching a blockade, it is considered a “prize.” The captured vessel and its cargo must be brought before a prize court (a national court of the capturing state) to determine the legality of the capture. If the capture is deemed lawful, the vessel and its cargo may be condemned as a “good prize” and become the property of the capturing state. If the capture is found to be unlawful, the vessel and cargo must be restored to their owners, often with compensation for damages.

Maritime Zones and Naval Operations

The legal framework of naval warfare also interacts with the law of the sea as defined by UNCLOS, which delineates various maritime zones.

  • Internal Waters and the Territorial Sea (up to 12 nautical miles from the baseline) are under the full sovereignty of the coastal state. Belligerent operations within neutral internal waters or territorial seas are generally prohibited and constitute a violation of neutrality. Belligerent warships have the right of innocent passage through the territorial sea of neutral states, but this passage must be continuous and expeditious and not prejudicial to the peace, good order, or security of the coastal state.
  • The Contiguous Zone (up to 24 nautical miles) allows coastal states to exercise control necessary to prevent infringement of its customs, fiscal, immigration, or sanitary laws.
  • The Exclusive Economic Zone (EEZ) (up to 200 nautical miles) grants the coastal state sovereign rights over natural resources and certain economic activities, but other states retain freedom of navigation and overflight. Belligerent naval operations, including exercises and passage, are generally permitted in the EEZ, but must not interfere with the coastal state’s resource rights or its sovereign rights over the continental shelf.
  • The High Seas are open to all states, and naval operations are generally permissible, subject to the rules of IHL.

The rules concerning naval mines are particularly important. Hague Convention VIII (1907) set early standards, prohibiting unanchored mines that do not become harmless after breaking their moorings, and requiring notification of minefields. While this convention is somewhat dated, its principles are largely considered customary international law. Modern practice emphasizes that mines must be directed against military objectives, must not be indiscriminate, and reasonable precautions must be taken to protect civilian shipping. Notification of minefields is crucial to ensure freedom of navigation for neutral vessels.

Conclusion

The rules of naval warfare constitute a vital, albeit complex, part of international law, serving to regulate armed conflict at sea and uphold humanitarian principles amidst the exigencies of military operations. These rules reflect a continuous effort to balance military necessity with the imperative to protect non-combatants and limit unnecessary suffering. Rooted in both customary international law and a series of foundational treaties, they provide a framework for distinguishing legitimate military targets from protected civilian entities, regulating the use of force, and outlining the rights and obligations of belligerents and neutral states.

Despite the comprehensive nature of these rules, the maritime domain presents unique challenges that continually test their application. The advent of new technologies, such as cyber warfare affecting naval systems, autonomous weapons, and dual-use vessels, demands ongoing interpretation and adaptation of existing legal norms. As naval operations grow in complexity and scale, the adherence to these rules becomes ever more critical to prevent escalation, protect vulnerable populations, and ensure accountability for violations. Ultimately, the effectiveness of the law of naval warfare hinges on its consistent application and respect by all parties involved in armed conflict at sea.