On 4 March 2026 a US Navy nuclear-powered attack submarine (SSN) sank the Islamic Republic of Iran Navy (IRIN) frigate IRIS Dena around 40 km off the southern coastline of Sri Lanka.

In a press conference on 4 March (US time) US Defense Secretary Pete Hegseth noted that this was the US Navy’s first sinking of an enemy vessel by torpedo since the Second World War.

While the Sri Lankan navy, coast guard and air force responded to a distress call received at 05.08 local time, IRIS Dena had already sank by the time the rescue effort reached it. A least 87 bodies were recovered by the Sri Lankan forces, who managed to rescue 32 Iranian sailors, although the ship’s crew at the time is believed to have numbered around 130.

The sinking of IRIS Dena was most certainly a devastating display of the global reach of the US military as the Trump Administration expanded the parameters of Operation ‘Epic Fury’, but was the Iranian frigate a legitimate target, or did the US Navy sink it simply because it could? Moreover, could the sinking even be considered a war crime? After all, the Iranian frigate was around 2,900 km from the southern coast of Iran at the time of its sinking and unlikely to enter the fray.

IRIS Dena, accompanied by the fleet supply ship IRIS Bushehr, had participated in India’s International Fleet Review 2026 and the multilateral naval exercise MILAN 2026 in the second half of February. These events were originally to also feature the Arleigh Burke-class destroyer USS Pinkney, but the US warship was called away just before they began.

IRIS Dena left the Indian port city of Visakhapatnam on 25 February. Hegseth noted in his press conference that the Iranian frigate “thought it was safe in international waters” when it was attacked.

A legal view

Warsight approached Nick Childs, Senior Fellow for Naval Forces and Maritime Security at the International Institute for Strategic Studies, for comment on the sinking of IRIS Dena in terms of international maritime law and rules of engegement.

Responding on 9 March, Childs stated, “There is, of course, much debate about the overall legality of the Israeli and US military action against Iran. Notwithstanding that, under the law of naval warfare, warships of states engaged in international armed conflict on the high seas or within the states’ own territorial waters are deemed lawful targets. That is not to say that the location of this sinking, remote from the main region of conflict, the manner of the attack and the relatively limited capabilities of the ship sunk, may make the attack appear shocking and unjustifiable to many. It has certainly provoked international protests, not least from the Indians, who had recently been hosting the vessel for exercises and a ceremonial fleet review. In particular, was there at least a less devastating way of putting this ship out of action?

“On the other hand, ships’ future movements and the threat they might pose in certain circumstances at some point cannot be predicted, nor the ability to locate and attack them again if necessary before that happened,” Childs continued, “so this incident will likely continue to provoke controversy and debate, rather in the manner of the sinking in the 1982 Falklands War of the Argentine cruiser General Belgrano by the British submarine HMS Conqueror.

However, Childs raised an important related issue in relation to the conduct of hostilities at sea: that belligerents are meant to assist the survivors of a sinking vessel, in so far as military circumstances permit.

“In this respect, submarines are considered a special case because of their vulnerability when surfacing and the fact that historically they have had very limited space and facilities to accommodate and care for survivors,” Childs pointed out. “Even so, there is the question of whether the US on this occasion at least notified others sufficiently – and perhaps specifically the Sir Lankans – in order for them to render assistance as effectively as possible. The speed with which the Sri Lankans were able to locate and recover survivors hints that perhaps they did, although initial reports suggested that it was a distress signal from the stricken ship that alerted them. But, in addition, in the actual circumstances of this conflict, and the fact that there appeared to be very little direct danger to the submarine, could it and its crew have done more directly to help in a rescue of survivors?

“More details may yet emerge about this incident,” Childs concluded. “Whether they will dampen or fuel the debate is another matter.”

The sinking of a navy

Unlike the Islamic Revolutionary Guard Corps Navy and paramilitary Basij Navy, which are more closely tied to the Islamic regime in Iran and largely operate small fast attack craft to conduct asymmetric operations, the IRIN has operated as a conventional national naval force. Prior to Operation ‘Epic Fury’, the IRIN operated around 70 surface combatants, including four frigates and five corvettes, as well as a ‘Kilo’-class attack submarine, a Fateh-class coastal submarine and 15 midget submarines.

At a briefing on 5 March at the headquarters of US Central Command (CENTCOM) in Tampa, Florida, the CENTCOM commander, Admiral Brad Cooper, noted that by that point US forces had sunk or otherwise destroyed over 30 Iranian ships. As well as IRIS Dena, this has included the Shahid Soleimani-class corvette IRIS Shahid Sayyad Shirazi, a Jamaran-class corvette, the Bayandor-class corvettes IRIS Bayandor and IRIS Naghdi, the drone/helicopter carrier IRIS Shahid Bagheri, the sea basing ship (converted oil tanker) IRINS Makran, and the IRIN’s only Fateh-class submarine.

Admiral Brad Cooper had stated during a briefing on 3 March: “We are sinking the Iranian navy: the entire navy.”

Unofficial wars

Although to all intents and purposes a war, the US Operation ‘Epic Fury’ against Iran, initiated on 28 February in conjunction with Israel’s Operation ‘Roaring Lion’, has not been officially declared a war. In fact, just like the US Navy torpedoing of an enemy vessel hasn’t happened since the Second World War, a US declaration of war hasn’t happened since against Japan on 8 December 1941 and against Germany and Italy on 11 December 1941. For example, US military operations since the Second World War against North Korea in 1950 to 1953, in Vietnam from 1961 to 1973, in Iraq from 1990 to 1991, in Afghanistan from 2001 to 2021 and in Iraq from 2003 to 2011 have never involved an official declaration of war.

Although President George W Bush declared a ‘Global War on Terrorism’ in the aftermath of the 11 September 2001 terrorist attacks on the United States, that was not put to the US Congress to vote on (it would certainly have passed) and was effectively the articulation of a more aggressive foreign policy to target Al Qaeda and its affiliates and supporters, rather than being directed at an individual sovereign state.

On 4 March a bipartisan resolution aimed at limiting President Trump’s ability to wage war in Iran failed in the US Senate by 47 to 53. The next day a similar vote in the House of Representatives failed by 212 to 219.

Torpedo attacks since 1945

Since the Second World War there have been very few naval engagements that have involved the sinking of an active warship by torpedo. As noted by Childs, on 2 May 1982 the Argentine cruiser ARA General Belgrano was sunk by the Royal Navy SSN HMS Conqueror during the Falklands War. This incident, which led to the deaths of 323 Argentine sailors, was controversial as it occurred outside an exclusion zone established by the UK government around the Falkland Islands, although its location was around 355 km south-southwest of the Falkland Islands – much closer to hostilities than IRIS Dena had been.

On 26 March 2010 the South Korean corvette Cheonon was sunk in the Yellow Sea, around 1.9 km off the southwest coast of South Korea’s Baengnyeongdo Island near the Northern Limit Line, by what is believed to have been a torpedo launched by a North Korean submarine, with 46 South Korean sailors killed.