A US submarine on Wednesday torpedoed Iranian warship IRIS Dena off the coast of Sri Lanka, outside its nautical boundary.The ship was returning after participating in the International Fleet Review and MILAN-2026, a multilateral naval exercise organised by the Indian Navy off Visakhapatnam last month.The sinking of the frigate, in which at least 80 sailors were killed, has widened the theatre of the war between US-Israel and Iran — beyond West Asia and the Gulf to India’s immediate neighbourhood.It also ignited a debate in India about maritime security in the Indian Ocean, a region where New Delhi maintains a significant naval presence.Several Indian Navy officers maintained that once a ship enters international waters, there is not much of a role any country can play in protecting it from an adversarial force. Here’s a look at what maritime law and conventions say.Conventions and lawsInternational maritime law falls under the purview of the United Nations Convention on the Law of the Sea (UNCLOS). The US is not a signatory to it.But it does not contain explicit provisions regulating the conduct of belligerent parties during armed conflict, as its primary focus is on peacetime governance.Story continues below this adInstead, the law of naval warfare operates in parallel with UNCLOS during conflicts. So, it doesn’t matter if the Dena was directly participating in the conflict. It was a warship of the Iranian navy and, therefore, a legitimate target.The UN Charter, too, addresses the use of force in international waters. Article 51 of the Charter, concerning the Right of individual or collective self-defence, provides an exception to the prohibition of the use of force as stipulated in Article 2 (4) of the Charter. Article 51 allows UN members to exercise their right to self-defence in the event of an “armed attack”.Alternatively, the Casebook of the International Committee of the Red Cross cites authorisation by the UN Security Council as grounds for a strike. While this measure dates back to the 1990 Gulf War, it may not be feasible. Chapter VII of the UN Charter, which outlines such authorisation, requires a majority vote from the council members with none of the permanent members — including the US — exercising their veto rights.The sanctions on the shipThe Iranian warship was sanctioned by the US Treasury Department in February 2023, along with executives of an Iranian drone company called Paravar Pars. In a statement, the US Department of the Treasury said that Iran was supplying UAVs for Russia’s combat operations to target critical infrastructure in Ukraine.Story continues below this adHowever, sanctions don’t mean that the ship cannot participate in a peacetime multilateral maritime exercise, but largely remains restricted to business or transactional activities.At the time of the attack, the ship is likely to have been armed with basic weapon systems, including close-in weapon systems and area defence systems.What experts sayVice Admiral G Ashok Kumar (Retd), former vice-chief of the Indian Navy and the country’s first national maritime security coordinator, pointed out that there is no restriction on a war zone in the maritime domain. “It is very sad for us that the incident happened as the ship was returning from an international event organised by the Indian Navy. But in the maritime domain, there is no restriction of battle or war zone, and the war started after they departed India,” he told The Indian Express.The former Navy officer added that the incident took place in the exclusive economic zone of Sri Lanka, and hence, they could quickly get into the search and rescue operation. “The fact that they were so close to the border might have meant that they were aware of some threat,” he said.Story continues below this adHowever, Rear Admiral Sudhir Pillai NM (Retd) argued in a Substack post that the use of force against a foreign warship on the high seas is presumptively unlawful unless clearly justified as self‑defence or as part of an armed conflict.In his Substack essay, he noted that a military strike in such circumstances could have been valid if it met the grounds for self-defence under Article 51 of the UN Charter.A senior Navy official said that the US submarine attack to down the Iranian warship in international waters while it was on a peaceful passage is a “big escalation” in the ongoing US-Israel-Iran war.“The ship was on a peaceful passage, and it wasn’t in the conflict zone. Even as the US has justified its action, this is a big escalation in the ongoing war,” said a senior official with the Indian Navy, who did not wish to be named.Story continues below this adHe further said that while warships have measures to counter torpedo attacks, the Iranian ship would have likely been taken by surprise.“They would not have expected an attack as they were on a peaceful passage and not in the conflict zone and were struck early morning. They may not have anticipated such an attack in international waters,” the official said.