On 4 March 2026, the US submarine torpedoed and sank the Iranian ship (IRIS Dena) when it was returning home after participating in the Milan multilateral naval exercise hosted by India. The incident occurred about 40 nautical miles off the coast of Sri Lanka, killing around 87 Iranian sailors.
Classification of Maritime Zones:
The United Nations Convention on the Law of the Sea (UNCLOS) governs control of the seas. The UNCLOS divides the maritime spaces into distinct zones with varying degrees of sovereignty and rights. The territorial sea extends up to 12 nautical miles from a coastal state's baseline, where the state exercises full sovereignty over its territorial sea, the airspace above it, and the seabed and subsoil beneath the waters. Beyond this lies the contiguous zone, extending up to 24 nautical miles, where the coastal state may enforce laws on customs, immigration, and security. Further out is the exclusive economic zone (EEZ), reaching up to 200 nautical miles, granting the coastal state sovereign rights over natural resources but not full sovereignty over navigation. Beyond this lies the high seas, which are beyond national jurisdiction, open to all states for navigation, fishing, and other lawful uses, governed by principles of freedom and collective responsibility.
Zone where the incident occurred:
The incident is alleged to have occurred in international waters. The term 'International Waters' is not defined in international law. It is an informal term that sometimes refers to waters beyond the “territorial sea” and is also used for the high seas. Technically, 'international waters' is a broader term used to refer to waters beyond a country's territorial sea. 'High seas' is a precise term that refers specifically to waters beyond 200 nautical miles.
This incident (the sinking of an Iranian ship) occurred approximately 40 to 44 nautical miles off the southern coast of Sri Lanka, within Sri Lanka's Exclusive Economic Zone (EEZ).
Article 56 of UNCLOS defines an exclusive economic zone (EEZ) as an area of the sea in which a sovereign state has exclusive rights regarding the exploration and use of marine resources, including energy production from water and wind. Article 58 of UNCLOS provides that all states enjoy the freedoms of navigation, overflight, and the laying of submarine cables/pipelines in the EEZ of another State. However, in exercising this freedom, States shall have due regard to the rights and duties of the coastal State.
Military Activities in the EEZ:
The status of military activities within EEZ remains one of the most contested issues in the law of the sea. While UNCLOS provides coastal states with sovereign rights over natural resources and certain economic uses of the EEZ, it does not explicitly prohibit foreign military operations in these waters. Major maritime powers, including the US, argue that the EEZ falls within the high seas regime for navigation and overflight, thereby permitting military exercises, surveillance, and submarine operations without coastal state consent. In contrast, some coastal states, such as China and India, argue that foreign military activities undermine their security and require prior authorisation. This divergence in interpretation has led to recurring disputes, highlighting the tension between freedom of navigation and coastal state security interests in international law.
Legitimacy of Target:
The Iranian warship, which sank, was allegedly not carrying any weapons, and hence the attack on the vessel was against international law. Principles of international humanitarian law clearly require that attacks be directed only at lawful military objectives. Although it is not a general rule that a ship coming for a military or naval exercise shall be weaponless. In fact, naval exercises are designed to test the operational readiness of ships, which includes their weapons systems, sensors, and crew capabilities.
However, IRIS Dena was allegedly following the 'peace protocol' of India's MILAN exercise. This protocol is an informal, safety and cooperation arrangement required participating foreign warships to operate in a non-combat configuration, with minimal or no active ammunition, to ensure safety during diplomatic events and exercises. Even during the sea phase, when operational drills and live-fire events occur, ammunition was strictly limited to what was required for specific drills, leaving ships vulnerable to attack.
Since the ship was not engaged in any act of war or hostilities, it cannot be considered a legitimate military target. Any attack directed against it must therefore be regarded as unlawful and a violation of international legal norms governing armed conflict.
Violation of International Law:
Even if one were to assume that the Iranian ship qualified as a legitimate target, International Humanitarian Law (IHL) lays down clear and well-settled rules governing the conduct of hostilities.
The Hague Convention (III) of 1907 relative to the Opening of Hostilities, to which the US has both signed and ratified, governs the legal requirements for commencing war and aims to prevent surprise attacks. Article 1 stipulates that contracting powers must not commence hostilities without prior and explicit warning (a declaration of war or an ultimatum). In this case, an Iranian ship was attacked by the US without any warning, showing a deliberate violation of Article 1.
Delving further into the law of naval warfare (a subset of IHL), which lays down specific rules governing the conduct of hostilities at sea. The conduct of operations at sea is regulated by the law of naval warfare, regardless of whether resort to force was lawful or a formal declaration of war or not. Since the law of naval warfare is lex specialis so it also takes precedence over the UNCLOS in case of conflict between the two.
The law of naval warfare also sets out obligations regarding the rescue of survivors. Article 18 of the Second Geneva Convention of 1949 (GC-II) mandates that after each engagement, parties to the conflict at sea shall, without delay, take all possible measures to search for and collect the shipwrecked, wounded and sick personnel, and to search for the dead and prevent their being despoiled. This obligation is non-discriminatory, protecting all combatants regardless of nationality, and extends to providing adequate medical care and protection against ill-treatment.
However, in the impugned incident, it was the Sri Lankan Navy that rescued the surviving sailors and retrieved the dead bodies, rather than the US Navy, highlighting a serious breach of obligations provided under the GC-II.
Furthermore, the US allegedly follows no rules of engagement; it only fights to win, and Victory matters only to them. By disregarding these established norms, the United States violated the rules of engagement under international law, rendering the attack unlawful. These are even bigger issues than sinking a ship, as they strike at the very foundation of international law and the rules that govern war itself, undermining the entire legal framework.
Indian Ocean Diplomacy:
India has continuously asserted its sovereignty in the Indian Ocean, positioning itself as the principal maritime power and “net security provider” in the region. The sinking of the Dena in the Indian Ocean has exposed deep fault lines in both international law and regional diplomacy. Colombo strongly objected, stressing that although the attack occurred in international waters, it was within Sri Lanka's EEZ, declaring that its seas and skies cannot be exploited by foreign powers to harm another nation.
India shall also strongly object to this incident, as it undermines regional trust and complicates the delicate balance of Indian Ocean diplomacy. For India, such violations of international naval warfare norms cannot go unanswered, and a firm objection is necessary to safeguard its credibility and influence in the region.
Author is a Faculty of Law at University of Delhi. Views are personal.