The reported sinking of the Iranian frigate IRIS Dena by the United States has quickly become more than just another geopolitical flashpoint. It has opened a deeper conversation about the legality of the use of force at sea. The attack occurred roughly 40 nautical miles south of Galle, Sri Lanka, when the vessel was returning from maritime engagements in India. Around 180 personnel were believed to be on board, with at least 87 confirmed dead, 32 rescued, and many others missing after the strike.
What makes the incident legally and diplomatically striking is the context of the ship’s voyage. IRIS Dena had recently participated in the International Fleet Review 2026 and the multinational MILAN 2026 in Visakhapatnam, events organized by the Indian Navy to promote maritime cooperation and interoperability among dozens of countries. The Iranian vessel had docked in India and departed after the exercise concluded in late February, sailing westward through the Indian Ocean before it was attacked. The fact that the ship had recently participated in a multinational naval engagement and was designed to build trust and cooperation among navies adds an unusual diplomatic dimension to the use of lethal force against it.
From a legal perspective, the incident sits at the intersection of three fundamental branches of international law is that the law governing the use of force, the law of the sea, and the law of naval warfare. Each of these frameworks attempts to regulate how states behave in maritime environments where sovereignty, security, and commerce constantly intersect.
Under modern international law, the starting point is the prohibition on the use of force contained in Article 2(4) of the United Nations Charter. This rule prohibits states from using force against the territorial integrity or political independence of another state except in narrow circumstances. Because warships represent the sovereignty of their flag state, attacking a naval vessel in peacetime is effectively equivalent to using force against the state itself.
The legal status of warships is further reinforced by the United Nations Convention on the Law of the Sea. Under UNCLOS, warships enjoy complete sovereign immunity when navigating international waters. They are not merely vehicles; they are floating embodiments of state authority. For this reason, the interference with or destruction of a foreign warship outside an armed conflict is considered an extremely serious act under international law.
Reports indicate that IRIS Dena was transiting through waters outside Sri Lanka’s territorial sea, likely within its Exclusive Economic Zone, but still part of international waters for navigation purposes. Under the law of the sea, an EEZ grants a coastal state economic rights over resources but does not restrict the navigation rights of foreign warships. Therefore, the Iranian vessel retained full navigational freedom in those waters.
The central legal question, therefore, becomes whether the United States could justify the strike under the doctrine of self-defence. Article 51 of the UN Charter recognizes a state’s inherent right to self-defence if an armed attack occurs. However, international law requires that the use of force in self-defence satisfy two conditions: necessity and proportionality.
These requirements were famously articulated in the Caroline Affair, which established that defensive force must be necessary, immediate, and leave “no moment for deliberation.” In other words, the threat must be imminent and overwhelming. If the Iranian warship was not actively threatening American forces or preparing an attack, the legal justification for torpedoing it becomes highly questionable.
The United States has reportedly framed the strike within the context of a broader regional conflict involving Iran. If the two states were indeed engaged in an armed conflict at the time, then the legal framework shifts from the law on the use of force to the law of armed conflict at sea. Under naval warfare rules, enemy warships may be treated as legitimate military targets.
However, even within armed conflict, the targeting of vessels is not unlimited. The laws of armed conflict require distinction, proportionality, and military necessity. Destroying a warship that is clearly part of the enemy’s navy would normally satisfy the requirement of distinction. Yet controversy may still arise if the vessel was not participating in hostilities or posed no immediate operational threat.
The incident also recalls several historical precedents in which maritime attacks sparked major legal debates. One of the most famous examples is the sinking of the Argentine cruiser ARA General Belgrano during the Falklands War. The British submarine attack occurred outside a declared exclusion zone, sparking international debate about whether the target posed a real threat at the time. The episode became a classic case study in naval warfare law and the interpretation of military necessity.
Another important legal precedent is the Corfu Channel Case decided by the International Court of Justice. In that case, the Court emphasized that maritime navigation and state sovereignty must be respected even in tense geopolitical environments. While the facts were different, concerning naval mines rather than torpedo attacks, the judgment reinforced the broader principle that states must exercise restraint in maritime operations affecting other sovereign vessels.
Even more striking is the Iran Air Flight 655 incident, where a U.S. cruiser mistakenly shot down an Iranian passenger aircraft over the Persian Gulf. Although the incident occurred under very different circumstances, it demonstrated how quickly maritime military actions can escalate into serious diplomatic and legal crises.
The sinking of IRIS Dena also carries symbolic significance because submarine attacks on surface warships have become extremely rare in modern times. Indeed, analysts have noted that this was the first time an American submarine has sunk an enemy ship with a torpedo since the Second World War, and one of the few such incidents since the sinking of ARA General Belgrano in 1982. This rarity highlights how exceptional the event is in contemporary naval history.
For the international community, the broader concern is the precedent such incidents may set. If warships travelling through international sea lanes after participating in cooperative naval exercises can be targeted without warning, it could erode the fragile norms that govern maritime conduct. The oceans are among the most shared spaces on Earth. Hundreds of warships from dozens of states routinely operate in the same waters from the Indian Ocean to the Pacific, and often in close proximity.
This is precisely why exercises like MILAN exist. They are designed not merely to train navies but to build confidence-building measures that reduce the risk of misunderstandings at sea. When a vessel that recently participated in such cooperation becomes the victim of lethal force, it undermines the very spirit of those initiatives.
Ultimately, the legality of the strike will depend on facts that may not yet be publicly known. Governments possess intelligence information that may alter the legal analysis. Yet international law demands transparency and justification when force is used against another state’s military assets.
If the attack cannot be convincingly justified under the framework of self-defence or armed conflict, it risks being interpreted as a violation of the fundamental prohibition on the use of force. And if that prohibition begins to weaken, the consequences for global stability, especially in crowded maritime regions could be profound.
In the end, incidents such as the sinking of IRIS Dena remind us how delicate the legal balance governing the world’s oceans truly is. Maritime spaces are shared by many states whose naval forces routinely operate in close proximity, making clear legal norms essential for preventing escalation. International law, particularly the framework established by the United Nations Charter and the United Nations Convention on the Law of the Sea exists precisely to manage such situations by requiring justification, proportionality, and accountability whenever force is used. As more facts emerge about the incident, the episode will likely become an important case study for scholars and practitioners of international law, illustrating how the principles governing the use of force and maritime navigation continue to shape state conduct in an increasingly complex strategic environment.
