The law of perfidy and ruses of war at sea

Abstract

The principle of distinction is a fundamental principle in the law of armed conflict, aimed at protecting civilians and civilian objects from direct attacks. Adherence to this principle is crucial for minimizing the humanitarian impact of warfare. Nevertheless, the longstanding practice of ruses of war, deeply embedded in customary international law, complicates the application of this principle in combat. While certain deceptive tactics are allowed, the line between acceptable ruses and forbidden acts of perfidy is often unclear, especially in naval warfare. The evolution of modern warfare technology further obscures these distinctions, raising important legal questions regarding their relevance today. This article critically examines the existing legal framework governing ruses and perfidy at sea and explores how these rules apply to contemporary naval operations. Finally, the article identifies key legal gaps and proposes pathways for reform to better align the law with current realities.

Introduction

A key objective of the law of armed conflict is to minimize collateral damage, particularly the suffering of civilians, who have historically borne the brunt of war. The principle of distinction was introduced to address this, ensuring that civilians and civilian objects are protected from direct attack. However, in the pursuit of victory, warring parties often resort to various means, including deception. While many forms of deception are illegal, some are permitted under international law. In naval warfare, however, the traditions and customs developed differently from those on land. An act deemed unlawful on land may not necessarily be illegal at sea. That said, the rapid advancement of technology in modern warfare has made some older legal frameworks seem outdated or impractical. This article examines the evolving application of the law of perfidy and ruses of war at sea in light of these developments.

The concept of perfidy and ruses of war

The principle of distinction is a cornerstone of the law governing armed conflict. It defines what can be lawfully targeted and what must be protected. Combatants and military objectives are legitimate targets, while civilians and civilian objects are not. This principle is crucial for safeguarding those not directly involved in hostilities. To enforce it, numerous rules and regulations have been established in international law over time. These include the requirement for soldiers to wear uniforms,¹ the obligation for combatants to carry arms openly,² the rule to distinguish combatants from protected civilians,³ the use of distinctive signals, emblems, and signs to signify the protected status of persons, platforms, or buildings, and the use of states’ flags during the war. Together, these measures help distinguish between lawful targets and those that must be shielded from attack.

Historically, parties to a conflict have often employed any means necessary to secure victory, with deception being one of the most effective tactics.⁴ In the current era, many forms of deception are used in warfare. Some of the most notable examples include the use of camouflage by soldiers, warships employing stealth technology,⁵ the deployment of dummy tanks during the World War II amphibious landing preparations,⁶ and the use of false intelligence information like the ‘mincemeat’ operation from the same era.⁷ These are considered lawful deceptions. However, there are also other forms of deception that are strictly prohibited. For instance, using the white flag to feign surrender before launching an attack, misusing the Red Cross symbol to gain a tactical advantage, or soldiers disguising themselves in civilian attire while engaging in combat are all illegal under international law.⁸

The concept of the prohibition of unlawful deception under Article 37 of the API is, at best, unclear. The API distinguishes between two forms of deception: ruses of war (ruse de guerre) and perfidy. Ruses of war are generally considered lawful, while perfidy is strictly forbidden. Ruses of war are generally considered lawful, while perfidy is strictly forbidden. Although international humanitarian law provides definitions for both, many ambiguities remain. Unlike the clear distinction between combatants and civilians—where civilians are defined by their non-combatant status—the line between ruses of war and perfidy is far less clear. The existence of specific treaty definitions for both suggests that not all deceptions outside the scope of perfidy are necessarily lawful ruses. Moreover, ruses themselves may be classified as either permitted or prohibited.⁹ For instance, the API commentaries, particularly in Paragraph 1515, emphasize the need to distinguish between lawful ruses, prohibited ruses, and acts of perfidy, indicating that some ruses of war may indeed be illegal.¹⁰

The API defines perfidy as acts that invite an adversary’s trust by leading them to believe they are entitled to protection under international law during armed conflict, only to betray that trust.¹¹ In contrast, ruses of war are described as acts intended to mislead an adversary or cause reckless actions, without violating any rules of international law.¹² Crucially, ruses do not rely on gaining the adversary’s confidence for protection under the law, which is why they are not considered perfidious. However, the distinction between ruses of war and perfidy can be subtle and is often subject to debate.¹³

Moreover, not all acts of perfidy result in war crime prosecutions. Article 37(1) of the API specifically prohibits perfidy that leads to the killing, injuring, or capturing of the enemy.¹⁴ There is ongoing disagreement among states regarding the causality element of perfidy. Some argue that the use of perfidy solely for capturing the enemy is not necessarily illegal,¹⁵ or at the very least, not regarded as a grave breach of humanitarian law.¹⁶ The Rome Statute of the International Criminal Court notably excludes the act of using perfidy to capture adversaries as a war crime,¹⁷ even though the International Committee of the Red Cross considers such acts as part of customary international law.¹⁸ A failed attempt at perfidy with the intent to kill, injure, or capture may still be illegal.¹⁹ Two identical actions may or may not qualify as perfidy, depending on the intent behind them. For example, feigning hors de combat (being out of combat) constitutes perfidy if the intent is to deceive and subsequently attack the enemy. However, if the aim is merely to delay an attack, even though it qualifies as perfidy, it is not prohibited under Article 37(1) of the API.²⁰

The law of perfidy and ruses of war unique in naval warfare: the use of false flag and the feigning of merchant vessel status

This section critically analyses the application of two unique aspects of perfidy and ruses of war in naval warfare: the use of false flags and the feigning of merchant vessel status. While these tactics have historically been considered lawful under the laws of naval warfare,²¹ advances in naval technology may challenge the reasoning behind their legality. Without a new treaty specifically addressing naval warfare, the definitive legality of these practices remains uncertain and will depend on future state practice.

The exceptionalism of naval warfare from other warfare

There were several attempts to modify the laws governing perfidy and ruses of war at sea.²² Harmonizing the law of war at sea with those on land may be important. Historically, battles were more compartmentalized—warships engaged each other at sea, while armies fought on land. Today, however, these boundaries no longer exist. Naval bombardments and air strikes from land-based aircraft often target maritime assets. Despite this evolution in warfare, the laws governing naval conflict, particularly those relating to perfidy and ruses of war, remain distinct and continue to play a critical role in regulating conduct at sea.

The traditions, customs, and rules governing warfare at sea have evolved separately from those for warfare on land. What may be considered illegal on land can be lawful at sea, and vice versa. A clear example of this is the law of prize: while the confiscation of private property is strictly prohibited in land warfare,²³ capturing and condemning an enemy’s merchant ship, even if privately owned, is fully lawful at sea—regardless of whether the ship directly supports its flag state.²⁴ There are no exceptions for merchant ships flying the enemy’s flag, even if they are owned by a citizen of a non-belligerent state.

Similarly, the use of perfidy and ruses of war differs between land and sea. On land, using false flags—whether the enemy’s or a neutral state’s—or pretending to be a civilian is strictly prohibited.²⁵ However, at sea, warships may use false flags, provided they lower them and raise their true flag before launching an attack.²⁶ Feigning merchant vessel status is also a recognized practice in naval warfare customs.²⁷ These rules did not develop in a vacuum. They have grown over time since the sailing ship era.²⁸ These customs did not develop in isolation; they have grown over centuries, dating back to the era of sailing ships. One key factor is the nature of the sea, where, unlike on land, there are few places to hide.²⁹ While soldiers can take cover in terrain or forests, warships, with the exception of submarines, have no such option.³⁰ To adapt, warships resorted to feigning civilian or neutral status as a means of concealment, so long as they revealed their true identity before engaging in hostilities.³¹ Over time, these practices became part of the customary international law unique to naval warfare.

The use of false flag in naval warfare

The use of false flag in naval warfare: Perfidy or ruses of war?

While the use of false flags is considered legal in naval warfare, a key question remains: does the use of false flags fall under the definition of ruses of war in the API or does it align more closely with the definition of perfidy? A closer examination of Article 37 of the API indicates that the use of false flags in naval warfare does not fit neatly into either category. Instead, it falls under the misuse of national emblems as outlined in Article 39 of the API. Specifically, Article 39(3) allows for the use of false flags in certain circumstances, exempting it from the restrictions found in Articles 37 and 39.

In land warfare, the use of any flag other than the enemy’s flag is prohibited at all times. However, the use of the enemy’s flag is permitted,³² provided it is not used during an attack,³³ or to shield, favour, protect, or impede military operations.³⁴ In such situations, the true flag must be displayed. In contrast, naval warfare allows the use of both enemy and neutral flags at all times, except during the act of attack.

False flag in naval warfare: past and present

In the age of sailing ships, and even during World War I and World War II, the use of a false flag as a ruse was a practical tactic. At that time, ships could only be detected visually, and naval engagements occurred at close range, either through hand-to-hand combat or relatively short-range cannon fire.³⁵ In this condition, the use of a false flag would deceive the enemy visually.³⁶ In this context, the use of a false flag was effective in deceiving the enemy, with the true flag being revealed only as the opposing forces came closer, catching the enemy by surprise.

Today, however, this traditional method of deception has become largely obsolete.³⁷ With the advent of over-the-horizon weaponry, radar, advanced communication systems, and satellite technology, the use of false flags in the traditional sense no longer holds the same tactical value.³⁸ Ships can now be identified through a variety of methods beyond flag recognition, including electronic emissions. In particular, the Automatic Identification System (AIS), which is now mandatory for merchant’s vessels, allows ships to be tracked globally in real time.³⁹ While the use of false AIS signals could theoretically complement false flag tactics, it is important to note that Article 39(3) of the API permits only the use of false flags as an exception in naval warfare. Despite the comparable role AIS plays today, this exception does not extend to electronic signals like AIS.⁴⁰

The feigning of merchant ship status

The feigning of merchant ship status: Perfidy or ruses of war?

Feigning the status of a merchant ship is a lawful tactic in naval warfare, permitted at all times except when an attack is imminent. Although this practice is legal, it is important to clarify which category it falls under perfidy or ruse of war. Based on Article 37(1) of the API, which addresses the feigning of civilian status, feigning as a merchant ship could be considered an act of perfidy. However, this type of ‘perfidy’ may be permitted under Article 39(3), even if it results in the killing, injuring, or capturing of the enemy.⁴¹

Lowering pennant: past and present

In the past, the use of pennants was crucial for identifying a warship, as it was often the only distinguishing feature separating warships from other types of vessels. At that time, the form and structure of warships were not significantly different from merchant ships. Visually, both types of vessels appeared similar, with hull colours providing no clear indication of a ship’s purpose. Even the presence of cannons on board was not exclusive to warships, as it was common for merchant ships to carry them as well. Privateers, operating legally, further blurred the distinction between merchant and war vessels.⁴²

Additionally, many warships were converted from merchant ships during times of peace, as maintaining a dedicated navy was costly and provided little economic return to the state. Evidence of this practice can be found in sunken warships that carried trade goods. Today, however, the distinction between warships and merchant vessels is much clearer, and the use of pennants as the sole means of identification has become largely obsolete.

In the modern era, the design of warships has evolved to be significantly different from that of merchant ships. Warships, particularly those designed for combat, are typically much sleeker and more agile, whereas merchant ships are built to carry large quantities of cargo. Another key distinction is the hull colour: warships almost universally feature grey hulls, while it is rare for merchant ships to adopt this colour. Additionally, the presence of armaments and advanced surveillance equipment aboard warships is a clear visual marker of their military purpose, a feature entirely absent from modern merchant ships.⁴³

Today, warships also commonly display a pennant number as part of their identification. This contrasts with the era of sailing ships, where lowering a pennant could be a deceptive tactic. In modern naval operations, however, such a simple act of deception would be ineffective in distinguishing between friend and foe. Unlike in the past, when close-quarters combat rendered the use of uniforms by naval crews essential for identification, modern naval warfare is generally conducted at long range, making the ship’s visual profile far more critical.

Following this reasoning, if the feigning of merchant vessel status (excluding protected vessels) is lawful in naval warfare, then it stands to reason that warships designed to fully mimic merchant vessels should also be permissible. A contemporary example of this is the concept of merchant missile ships,⁴⁴ which retain civilian status in peacetime but can legally be converted to warships during wartime, provided the legal requirements for conversion are met.⁴⁵ The use of Q-ships during the World Wars further illustrates this practice.⁴⁶ Q-ships not only employed deceptive flags and ensigns but also incorporated fake superstructures, such as false funnels and hidden armaments concealed within crates. However, it is important to note that Q-ship engagements typically occurred at close range, where visual deception was crucial, such as ramming U-boats or engaging with smaller or medium-calibre guns within visual range.⁴⁷

Hoisting the ensign

When the time comes to launch an attack, a feigning warship is required to hoist its ensign and cease all deceptive measures.⁴⁸ The question that arises is whether, beyond hoisting the ensign, there is an obligation to transmit accurate electronic data, or if simply discontinuing the false signals is sufficient.⁴⁹ While this may seem like a narrow and technical issue, it has significant tactical implications. In the case of Q-ships from the past, aside from raising their ensigns, little was done to fully reveal their true identity as enemy warships. For example, the fake merchant ship names on their hulls often remained. Modern warships feigning as merchant vessels actively simulate their civilian status, including transmitting electronic signals such as AIS and engaging in radio communications typical of merchant ships. A warship disguised as a fishing vessel could similarly use a fishing vessel transponder, known as the Vessel Monitoring System (VMS),⁵⁰ and even deploy nets and deceptive lighting to imitate a ‘vessel engaged in fishing activity’.⁵¹ Legally, when the time comes to attack an enemy, particularly with long-range missiles targeting beyond the horizon, the warship must display its true flag. However, given that the target is far out of visual range, the only methods for detecting the attacking vessel are through radar, radio signals, and its electronic emissions via AIS and VMS. This raises the question of whether merely hoisting the ensign suffices when electronic deception continues to play a key role in the engagement.

Traditionally, revealing a warship’s true identity has been understood as simply hoisting its ensign. However, in modern warfare, hoisting the ensign alone may be insufficient to disclose the ship’s true status, especially when the target is far away. In such cases, hoisting the ensign would not effectively communicate to distant targets that the ship is a warship. There is no question that a feigning warship must cease all false electronic emissions, radio signals, and the deceptive behaviour of mimicking a fishing vessel when launching an attack. The key question is whether the ship must also broadcast its true identity, for instance, by transmitting its warship status through AIS or other identification systems.

If the logic of hoisting the ensign in the past is adapted to modern warfare, it would imply that the attacking ship must clearly signal its identity to the enemy, giving them an opportunity to defend themselves. In today’s context, failing to transmit the ship’s true identity via AIS, radio, VMS, or other communication channels is equivalent to not hoisting the ensign at all. This omission creates a deceptive situation. Simply stopping false transmissions, without actively providing accurate information, leaves the enemy uncertain about whether the ship is a warship or a fishing vessel.

One could argue that radar detection of the missile launch would reveal the ship’s identity when transmission regulations are ‘on’. While this is possible, radar alone may not provide sufficient clarity. At a distance, a radar image could suggest the presence of two vessels: one being the warship that launched the missile, and the other an innocent fishing vessel nearby. This uncertainty forces the enemy to factor in potential collateral damage to the fishing vessel before launching a counterattack on the warship. On the other hand, if the feigning warship not only hoists its ensign but also transmits accurate electronic signals, there would be no doubt on the enemy’s radar that the ship is a warship. This would eliminate any delay in the enemy’s decision to counterattack.

In practice, a feigning ship can transmit multiple electronic signals simultaneously, creating further confusion. When it comes time to launch a missile, the ship may hoist its true colours and transmit its true identity as a warship while continuing to emit false signals, giving the impression that there are one or more fishing vessels near the warship. This deception can be amplified by the use of chaff, creating multiple false radar contacts on the enemy’s radar. As a result, the enemy’s radar operator may mistakenly believe that there is a fishing vessel in close proximity to the target.

It is important to note that in this scenario, the warship is not engaging in perfidy by pretending to be a fishing vessel. Rather, it is creating a false radar image to suggest the presence of civilian vessels in the area. This differs from directly feigning to be a fishing vessel. This form of naval deception could be likened to the land-based deception used at the Bankstown fighter aerodrome in Australia during World War II, where the site was designed to resemble a rural town.⁵²

In this situation, the enemy must take the ‘precaution in attack’ principle into account before launching a counterattack, in order to avoid harming the ‘imagined’ fishermen—who would be protected civilians—and destroying the ‘imagined’ fishing vessels, which would be considered civilian objects.⁵³

The use of false flag or the feigning of merchant ships during naval bombardments on land

While the use of false flags is permitted in naval warfare, except when launching an attack, an important question arises: what about during naval bombardments targeting land from warships? This issue remains largely unexplored. The API, the San Remo Manual on International Law Applicable to Armed Conflicts at Sea (1994), and the Newport Manual on the Law of Naval Warfare (2023) are all silent on this matter. The dichotomy between the laws of perfidy and ruses of war on land and at sea does not fully account for the integrated nature of modern battlefields. When hostilities occur at sea, the laws of perfidy and ruses at sea apply. On land, however, the use of false flags or feigning civilian status is prohibited. But what happens when one party is at sea and the other on land? Which rules governing ruses and perfidy should apply?

For the party on land, the rules require maintaining true identity at all times.⁵⁴ In contrast, the party at sea has more flexibility, being allowed to disguise itself as a neutral warship or a merchant vessel until the moment of attack. This discrepancy becomes significant when one side has dominant naval power and the other has a strong land-based force. Warships can easily verify targets on land through advanced technologies like satellite surveillance, which can monitor and confirm the identity of land targets without the need for false flags. On the other hand, identifying a maritime target is more challenging. A warship can disguise itself as a merchant vessel or neutral warship, and even though it may be monitored by satellite, its use of deceptive tactics—such as altering its structure, acoustic signals, electronic transmissions, and lighting—makes verification much more difficult.

The rules governing the use of false flags in naval warfare were developed primarily for engagements between warships, not as part of a naval bombardment strategy. In the era of sailing ships, it was far more challenging for warships to bombard coastal areas than it is today. The short range of cannons required ships to approach much closer to the shore to launch an attack. The range of a ship’s cannon was typically less than 3 nautical miles, and the cannons aboard ships were generally smaller than those used on land. Additionally, coastal fortresses had the advantage of higher ground, allowing their cannons to fire farther than those on warships. In contrast, land-based cannons were generally larger and had greater range, posing a significant threat to approaching warships. The ship’s buoyancy limitations restricted the number of heavy cannons it could carry. The heavier the cannons, the greater the risk of the ship capsizing. Additionally, shallow waters often hindered ships from getting close enough to shore. There are historical examples where, under certain conditions, warships were able to attack coastal areas, particularly when the waters were deep enough for them to approach.⁵⁵ However, using a false flag in these cases would have been futile, as the coastal defenders were already aware of the ship’s hostile intent or actions.⁵⁶

In the modern era, a ship’s identity is not solely determined by its ensign. Electronic identification, such as the widespread use of AIS, now plays a crucial role in ship recognition. Many institutions, including port authorities, rely on AIS for identification rather than solely depending on the flag a ship flies. As a result, the relevance of false flag rules in cases where the target is on land is questionable. Neither the San Remo Manual of 1994 nor the Newport Manual of 2023 distinguish between targets at sea and those on land. The lex lata on this issue will ultimately be shaped by future state practices and the recognition of those practices by other states.

The law of perfidy and ruses of war for submarine warfare

One of the most effective forms of deception on land is the use of camouflage, allowing combatants to blend into their surroundings. At sea, submarines achieve a similar effect by submerging, effectively blending into the waters and becoming relatively undetectable by the enemy. As discussed earlier, the historical development of the legal recognition of using false flags in naval warfare stemmed from the fact that surface warships had no natural cover at sea, unlike land-based combat, where terrain, jungles, and other natural features provide hiding places. Submarines, however, can remain hidden underwater for extended periods, raising the question of whether submarines have the right to feign while submerged.⁵⁷ In other words, is it lawful for a belligerent submarine to disguise itself as a neutral submarine?

Submerging provides submarines with a significant tactical advantage, allowing them to remain hidden from enemy surface ships and aircraft. When compared to military aircraft engaged in naval warfare, submarines arguably require their own set of rules regarding perfidy and ruses of war. Unlike surface warships, military aircraft, whether confronting enemy planes or warships, do not have the option to use false flags as a deceptive tactic.⁵⁸ Therefore, in the absence of state practices to the contrary, it seems necessary to establish specific legal regulations governing perfidy and ruses of war at sea that apply uniquely to submarines.⁵⁹

While the very nature of a submarine is inherently deceptive, submarines are capable of employing additional forms of deception. Unlike surface warships, which often rely on false flags, submarines operate in an environment where such tactics are impractical. A flag, real or false, cannot be flown underwater. However, submarines possess their own unique ‘fingerprint’—their acoustic signature. This signature, generated by propulsion systems, sonar, generators, machinery, and other equipment, serves as a submarine’s distinctive identity.⁶⁰ Most navies maintain extensive libraries of acoustic signatures from submarines around the world, allowing for identification.⁶¹

Typically, submarines reduce their noise to evade detection by enemy sonar. Yet, it is not beyond the realm of possibility for a submarine to mimic the acoustic signature of another, creating potential confusion between enemy and neutral submarines. This raises important legal questions: Is it lawful for a submarine to use the acoustic signature of another vessel? Should a submarine, like a surface warship hoisting its true flag before an attack, be required to reveal its true identity by changing its acoustic signature during combat? Although such a change might only be feasible in a dry dock, a more practical method for mimicking another submarine’s acoustic signature would be through the use of an underwater unmanned vehicle. The legal and strategic implications of these tactics warrant closer examination, particularly as mistaken identity in submarine warfare could have serious consequences for surface vessels.

Nowadays, drone technology has advanced significantly, including the development of underwater drones, also known as unmanned underwater vehicles (UUVs). UUVs serve a variety of purposes, one of the most promising being their application in the defence sector. In particular, UUVs can be used to emit the acoustic signature of a submarine, effectively acting as ‘dummy submarines’.⁶² This is especially valuable in anti-submarine warfare drills for surface warships and aircraft. In such exercises, a UUV can be programmed to mimic the acoustic signature of a submarine and then deployed underwater. Surface warships and military aircraft equipped with underwater surveillance systems attempt to locate the UUV, and once detected, they launch simulated attacks using anti-submarine weaponry. While the use of ‘dummy submarines’ in training exercises poses no legal or ethical concerns, the question arises: what happens if this technology is employed in a real armed conflict? The strategic implications of using UUVs to deceive an adversary in active warfare introduce complex legal and ethical dilemmas that warrant careful consideration.

Naval intelligence agencies from modern navies routinely collect data on the acoustic signatures of submarines from various states, which is crucial for identifying these submarines when detected. UUVs have the capability to emit false acoustic signatures, potentially mimicking submarines from different states. In a conflict, an enemy might deploy a UUV to emit the acoustic signature of a neutral state’s submarine, attaching it to their own vessel. In this way, the enemy submarine uses the UUV as an ‘underwater false flag’.⁶³ The key question is whether this tactic is lawful. Can a submarine use a false acoustic signature in the same way a surface warship can use a false flag?

Historically, the use of false flags has been limited to surface warships. When a submarine surfaces, it is legally permitted to display a false flag in the same manner as surface ships. Similarly, a surfaced submarine may legally feign as a merchant ship by transmitting false AIS data or employing other deceptive measures. However, the question remains: does a submarine have the right to engage in deception while submerged? Have there been any instances of state practice allowing the use of a false flag by a submarine underwater? And if such practices exist, has there been acquiescence or recognition from other states?

When a submarine is preparing to launch an attack on a surface ship, is it required to cease all deceptive measures and reveal its true identity?⁶⁴ This question arises because the surface ship may have already identified the submarine but mistakenly believes it to be neutral due to the submarine’s use of decoys or other methods to falsify its acoustic signature. Even if the surface ship attempts to verify the submarine’s identity through Underwater Telephony (UWT) or Gertrude, should the submarine be obligated to disclose its true identity just as it launches its torpedo?

Article 39(3) of the API defers to other rules of international law regarding the use of false flags in naval warfare, thereby exempting it from the category of prohibited perfidy. Unfortunately, there are no treaty provisions that specifically address the use of perfidy and ruses of war at sea. Although efforts to codify customary international law on these practices into treaty form have been made, they never materialized. In the modern era, two key manuals offer guidance on this issue: the San Remo Manual and the Newport Manual. However, these manuals are not legally binding, as they are not treaties, although many of the rules they contain reflect customary international law.

Both the San Remo Manual of 1994 and the Newport Manual of 2023 address the right of warships to use false flags, but they do not distinguish between surface warships and submarines.⁶⁵ Without clear state practice, it is difficult to conclude that the rules governing false flags apply equally to submerged submarines. The term ‘warship’ should not be interpreted to include all types of warships, including submarines, without further consideration of customary international law. To the best of the author’s knowledge, there is no established practice of submarines engaging in perfidy or ruses of war while submerged.

While submarines were widely used during both World Wars, the technology of the time did not support the use of false acoustic signatures. The invention of UWT, or Gertrude, only occurred in 1945—2 years after the peak of German U-boat activity during World War II.⁶⁶ Post-World War II, the use of submarines in warfare has been relatively rare; and in the notable case of the sinking of the General Belgrano by a British submarine, no ‘false flag’ tactics were employed.⁶⁷ Therefore, the positivist law of this topic will be determined by the practices of states and the acceptance of those practices by other states.

Ruses and perfidies of combat divers

Combat divers are unique in naval warfare. Naval warfare normally uses platforms such as warships, military aircraft, or auxiliaries to conduct hostilities. Combat divers, on the other hand, are relatively the same as combat soldiers on land. They do not depend on platforms to conduct hostilities.⁶⁸ Since naval warfare usually uses platforms for war, the principle of distinction at sea evolves differently from its counterpart on land. On land, uniforms play a critical role in distinguishing combatants, and wearing enemy uniforms during an attack is strictly prohibited.⁶⁹

At sea, however, this distinction is less rigid. Crews aboard warships are not bound by uniform requirements during engagements and may wear civilian attire or even enemy uniforms.⁷⁰ This flexibility stems from the fact that modern naval warfare is typically conducted at a distance between platforms, rather than through direct, hand-to-hand combat between warship crews, as was common in the past. The principle of distinction in naval warfare focuses primarily on the status of the platform itself. Civilians aboard warships are not factored in the calculation of potential collateral damage, nor are they considered in the proportionality assessment of whether the military advantage gained from an attack outweighs the potential harm to civilians.

Combat divers, though operating in the maritime domain, are not tethered to any platform, raising important questions about which laws of perfidy and ruses of war apply to them.⁷¹ Can they wear civilian diving suits or even enemy uniforms as they approach an enemy port? Could they disguise themselves as civilian divers while operating behind enemy lines? It is worth noting that, on land, soldiers found behind enemy lines in civilian attire or enemy uniforms face charges of espionage, as seen in Ex Parte Quirin⁷² and the Osman Case.⁷³ The legal implications of such actions at sea require further scrutiny, particularly in light of how the laws of perfidy and ruses of war may evolve in the context of maritime special forces.

The use of uniforms is crucial for upholding the principle of distinction. Wearing distinctive attire, even without badges, generally meets the requirement of differentiating not only between combatants and civilians but also between friendly and enemy forces.⁷⁴ In the case of military divers, however, the significance of uniforms diminishes, as they are concealed underwater. To date, there are no known cases of military divers being accused of perfidy or espionage behind enemy lines. Typically, military divers wear specialized swimsuits that sufficiently distinguish them from civilians.

Carrying arms openly is a key element in distinguishing combatants from protected civilians.⁷⁵ However, in the case of combat divers, even when they carry their underwater assault rifles openly, it is difficult for observers to differentiate between these weapons and the spears or harpoons used by traditional divers—unless observed at close range.

A more pressing question arises when combat divers simulate traditional divers. Is this considered an act of perfidy? There are no treaty provisions addressing this issue, and neither the San Remo Manual of 1994 nor the Newport Manual of 2023 provide guidance. This scenario has not been discussed, likely due to the general assumption that people cannot dive for extended periods without breathing apparatus. However, in regions such as Southeast Asia, the Bajau people—also known as sea gypsies—are capable of swimming and diving for long periods without any equipment, as their livelihood depends on the sea.⁷⁶ If a war at sea arises in this region, applying the principle of distinction between military divers and the Bajau people becomes crucial.

Perfidy and ruses of war at sea in relation to the law of neutrality at sea

There are several scenarios that connect the law of neutrality at sea with the law of perfidy and ruses of war at sea. While the use of a false flag is generally considered lawful in naval warfare, its use to deceive neutral states raises questions about whether it could be classified as prohibited perfidy. According to Article 37(1) of the API, perfidious acts are prohibited when they directly cause death, injury, or capture of an adversary. Since neutral states are not adversaries in conflict, their involvement might fall outside the scope of perfidy as defined by the API.⁷⁷ Nevertheless, the burden of a ‘violation by using a false flag’ in neutral waters ultimately falls on the enemy state. Such actions may not constitute prohibited perfidy under the API, but they could still represent a breach of neutrality through the use of false flags or feigned merchant ship identities. Below are some possible scenarios that explore these complexities.

The 24-h rule

The 24-h rule is part of the law of neutrality at sea, designed to prevent belligerent states from using neutral territory or waters as a sanctuary.⁷⁸ While simply violating this rule does not constitute perfidy, it clearly breaches the law of neutrality. However, belligerent warships may attempt to circumvent the rule through deception, raising the question: what if the use of a false flag is intended to deceive neutral states rather than the enemy?⁷⁹ Is it lawful for a warship to disguise itself as a neutral vessel?

If there is a violation of neutrality, it is the responsibility of the neutral state to address it. Failure to enforce neutrality could lead the aggrieved belligerent state to take action to stop the breach.⁸⁰ However, if no belligerent state reports that an enemy warship has violated neutrality by disguising itself as a neutral merchant ship or using the flag of a neutral warship, the neutral coastal state may find it difficult to prevent such violations. This is because there would be no apparent reason for the neutral state to impede the innocent passage of a ship flying another neutral state’s flag.

Another scenario involves the use of the UN flag by belligerent states during an armed conflict in which the UN itself is a party. In such a case, the UN flag is not protected from being used deceptively, allowing belligerent warships to feign as UN vessels.⁸¹ However, this raises the question: can a feigning warship use neutral waters as a sanctuary by staying there for more than 24 h? It is important to remember that all UN Member States, including the state whose waters the feigning warship is stationed in, are obligated to support UN Security Council resolutions, particularly those enacted under Chapter VII of the UN Charter.⁸²

The impartial closure of neutral waters

Neutral coastal states have the right to close their waters—territorial seas and, where applicable, archipelagic waters—to belligerents, provided the closure is enforced impartially.⁸³ However, this raises the issue of belligerent warships using false flags or feigning as merchant vessels to conduct innocent passage through closed neutral waters. By posing as neutral ships, belligerent warships could easily traverse these waters undetected. Belligerent warships feigning neutral vessels would not be subject to inspection by the neutral coastal state unless there were clear signs of violation, as stipulated in section 3 of the 1982 United Nations Convention on the Law of the Sea (UNCLOS). According to UNCLOS, coastal states ‘cannot hamper the innocent passage of foreign ships through the territorial sea except in accordance with this convention’.⁸⁴ Expecting coastal states to identify belligerent warships disguised as neutral vessels—whether as warships or merchant ships—without specific indicators of deception is highly unrealistic.

Surface passage of belligerent submarines in neutral waters

A related question arises regarding the use of a false flag by a submarine during innocent passage through neutral waters when the submarine is required to surface and fly its flag.⁸⁵ In times of armed conflict, a submarine’s position is typically kept secret. However, if circumstances force a submarine to pass through neutral waters by surfacing, it might employ a false flag to deceive both the neutral coastal state and the enemy.⁸⁶ Is this use of a false flag permissible while the submarine is surfacing? While surfaced submarines are generally subject to the same rules as surface warships, innocent passage through neutral waters requires that the submarine surface and signal that it poses no threat to the coastal state.⁸⁷ In this context, can the submarine lawfully engage in deception? This scenario may fall outside the scope of prohibited perfidy under Article 37(1) of the API, as it does not involve the killing, injuring, or capture of anyone, and the target of the deception is not the enemy state.

The use of false flag by merchant ships

The Newport Manual of 2023 clarifies that the use of false flags by merchant vessels to evade capture is permissible.⁸⁸ However, there remains no clear evidence that this practice has been fully incorporated into customary international law.⁸⁹ If this practice becomes widespread, it is likely that, during wartime, enemy merchant ships will routinely adopt neutral or even other belligerents’ flags to avoid capture. This raises a significant issue, particularly when conflicts are undeclared or of low intensity, as it becomes difficult to determine when the use of false flags by foreign vessels is legally justified. The ambiguity surrounding the commencement of the law of neutrality complicates the matter further.⁹⁰ It is unclear when the use of false flags is permitted and when it should cease. If the neutral state whose flag is being used does not recognize the state of war of the ship’s country, it will operate as though it is peacetime, applying the law of the sea. The law of the sea prohibits vessels from claiming dual nationality, treating such ships as stateless. The legality of this practice can only be judged retrospectively. Another critical question is whether all states accept the use of false flags by merchant ships to evade capture. Each state has the authority to regulate this practice through its domestic laws. If the neutral state whose flag is being used does not agree with this rule, it may assert its right to board the ship, as the use of its flag implies that the ship is prima facie under its jurisdiction.

Conclusion

Significant legal gaps remain in the application of the law of perfidy and ruses of war at sea in the modern era. While the use of false flags and the feigning of merchant ships are considered legal in naval warfare, there are critical questions that need further clarification. For instance, whether other deceptive measures—such as pretending to be the warships of neutral states—are lawful, and if so, whether an attacking warship must merely cease emitting false electronic signals or actively disclose its true identity.

The application of false flags in naval bombardments targeting land forces also requires closer examination, as current rules were developed primarily for ship-to-ship combat. Submarines present a unique challenge, given their ability to remain concealed underwater. Whether they can legally adopt the acoustic signatures of neutral submarines remains an open question.

Similarly, the rules governing perfidy and ruses of war at sea may not be entirely appropriate for combat divers, who operate more like individual combatants than traditional warship crews. In the context of maritime warfare, where the law of neutrality plays a critical role, it is essential to determine whether deceiving neutral states through perfidious acts could fall under prohibited perfidy.

Finally, regarding merchant ships, there is no broadly accepted customary international law that legitimizes the use of false flags to evade capture. The lack of consensus in this area underscores the broader need for further legal development and clarification in modern naval warfare, particularly as new technologies and tactics continue to evolve.

Acknowledgements

The authors thank David Letts for his comments on the initial draft of this article.

Funding

Funding support for this article was provided by the LPDP (2021112227733).

Footnotes