When the world's at stake,
go beyond the headlines.

A U.S. submarine recently sank an Iranian warship in one of the world’s busiest maritime corridors. Many overlooked the incident’s significance. Others misunderstood what had occurred.

The sinking of the IRIS Dena in the Indian Ocean, roughly 40 nautical miles off Sri Lanka’s southern coast, was a graphic reminder of the brutality of war on the high seas. Footage of the ship’s stern exploding and images of Sri Lankan Navy personnel rescuing survivors quickly circulated online, prompting fierce debate about the legality of the strike and the nature of combat at sea. It also exposed a broader problem. Naval warfare remains poorly understood outside naval circles.

The IRIS Dena was a lawful target under the law of naval warfare. It is also long recognized in international law scholarship that modern submarines cannot safely surface to rescue survivors. Their obligation is instead to notify appropriate vessels or authorities so that rescue can be undertaken as soon as practicable. Yet, misunderstandings of these aspects dominated early reporting.

The wider conflict between the United States, Israel, and Iran had begun on Feb. 28, only days before the IRIS Dena was sunk. Within days, degrading Iranian naval capability had emerged as a key operational objective. The sinking of the IRIS Dena did not occur just anywhere. It took place along one of the world’s most consequential trade routes in the Indian Ocean, a corridor critical to global energy flows and to China’s economic security. The engagement was not only a tactical success but also a broader strategic signal.

In the immediate online reaction to the sinking, narrative often overtook fact. So, what happened in the Indian Ocean, and why was it lawful?

Iran’s Naval Presence Beyond the Gulf

Just after 5 a.m. on March 4, approximately 40 nautical miles south of Galle, the IRIS Dena was sunk by a Mark 48 heavyweight torpedo fired from the USS Charlotte. The ship was a relatively new multi-role frigate, launched in Iran in 2015 and commissioned into service in 2021. One of seven Moudge-class frigates operated by the Iranian Navy, the vessel displaced roughly 1,500 tons, placing it closer in size to a corvette or offshore patrol vessel than a traditional frigate. Despite its modest dimensions, it carried a crew comparable to many larger surface combatants, with a normal complement of around 140 personnel. Reports suggest between 130 and 180 personnel were onboard when the ship was sunk.

Despite claims that the vessel was unarmed, the IRIS Dena was reasonably capable for its size, though no realistic match for a nuclear-powered attack submarine. Open source assessments indicate the ship carried a 76 mm naval gun, surface-to-air missiles, anti-ship missiles, triple 324 mm torpedo launchers, and a hull-mounted sonar. Warships rarely transmit on hull sonar while transiting unless a submarine threat is suspected. In practice, this distinction is unlikely to have mattered. The attacking submarine likely fired from well beyond detection range. The IRIS Dena would therefore have had little warning of the strike, and even if contact had been gained, its lightweight torpedoes would have offered only limited defensive options.

Since commissioning, the IRIS Dena had become a visible instrument of Iranian naval diplomacy. A 2023 deployment with the converted tanker IRIS Makran was designed to signal Iran’s intent to operate well beyond the Persian Gulf and build a more credible blue water naval capability. In February 2026, the frigate was operating in the Indian Ocean after participating in India’s International Fleet Review in Visakhapatnam and the multilateral Exercise MILAN alongside the Iranian landing ship IRIS Lavan. It was reportedly transiting back toward Iran when attacked. Another Iranian tanker, the IRIS Bushehr, was also operating in the region in support of the task group.

Misinformation in the Maritime Domain

Shortly after the sinking, inaccurate reports circulated claiming that the IRIS Dena had been unarmed, with some suggesting that participation in India’s International Fleet Review required vessels to sail without weapons. The fleet review and Exercise MILAN brought together more than 60 ships and aircraft from around 70 countries. While there would have been guidelines governing conduct during the events, there was no requirement for participating warships to be unarmed or to sail without ammunition. In years of planning and participating in multinational naval exercises across the Indo-Pacific, such a requirement is not one I have encountered. The sea phase of Exercise MILAN also appears to have included live-fire activities, further undermining claims that participating vessels were required to be unarmed.

Although the precise loadout of the IRIS Dena cannot be confirmed from open sources, it would be highly unusual for a warship to transit the Indian Ocean to a multinational exercise without its normal weapons and ammunition, particularly at a time of heightened tensions between Iran and the United States. For a commanding officer to deploy and remain at sea in such circumstances without an appropriate combat load would raise serious professional questions. More broadly, the episode illustrates how quickly misinformation can take hold in the modern information environment, especially when naval operations occur far from public view and public understanding of maritime warfare remains limited.

A Submarine Engagement at Range

The IRIS Dena was reportedly seeking permission to enter Sri Lankan waters when it was engaged by the USS Charlotte. Washington has not publicly outlined the sequence of the engagement. Media outlet Iran International claimed that one sailor who died in the sinking contacted his parents to say the vessel had been warned by American forces to abandon ship. This account has not been verified and would be unusual in the context of a submarine engagement.

The attacking submarine is understood to have fired a Mark 48 torpedo, detonating beneath the stern of the IRIS Dena. Following the sinking, the Sri Lankan Navy conducted rescue operations with initial reports indicating the recovery of 32 survivors and 87 bodies, with a further 61 crew members still missing. The speed of the response suggests local authorities may have been alerted before the vessel sank. Some reports indicate the ship transmitted a distress call shortly after 5 a.m., while the U.S. Navy has confirmed that it notified Sri Lankan authorities of the incident.

The engagement also attracted attention in Australia after the prime minister confirmed that three Australian sailors were embarked on the U.S. submarine at the time, though they were not involved in the strike.

What the Law of Naval Warfare Permits

The sinking also prompted debate about neutrality and the legal implications of Australian sailors embarked on the attacking submarine. These issues warrant separate consideration but do not alter the lawfulness of the strike. The more immediate questions are straightforward: Was the Iranian warship a lawful military target, and was there an obligation to rescue survivors? Established principles of the law of naval warfare provide clear answers.

The law of naval warfare forms part of the broader law of armed conflict and applies specifically to the conduct of hostilities at sea. Under this framework, warships of a belligerent state are lawful military objectives by virtue of their status. Once an international armed conflict exists between states, their military vessels may be attacked wherever they are encountered on the high seas or in belligerent waters, subject to the rules governing the conduct of hostilities. In practical terms, this means that a vessel such as the IRIS Dena could be lawfully targeted because it formed part of the armed forces of a belligerent state and contributed to its military capability.

The fact that the ship was operating in the Indian Ocean rather than close to Iranian waters does not alter this legal assessment. Naval warfare has long been conducted across vast maritime spaces, and belligerent warships have historically been engaged wherever they are found on the high seas.

A second issue concerns the obligation to rescue survivors. The law of armed conflict at sea recognizes a duty to assist those who are shipwrecked, wounded, or otherwise no longer able to take part in the fighting. This obligation, however, is not absolute. Commanders are not required to undertake rescue operations where doing so would place their own vessel or mission at serious risk. This qualification is particularly relevant in submarine warfare. Unlike surface combatants, submarines rely on stealth for survivability. Surfacing to conduct rescue operations may expose them to detection and attack.

In such circumstances, humanitarian obligations may instead be met by alerting nearby authorities or other vessels capable of conducting rescue operations. Reports that the United States notified Sri Lankan authorities of the sinking of the IRIS Dena would be consistent with this obligation.

In naval warfare, the duty to assist survivors must ultimately be balanced against the operational realities of fighting at sea, a limitation generally recognized in international law.

Conflict Along Critical Sea Lines of Communication

The sinking of the IRIS Dena was more than a tactical success in the war between the United States, Israel, and Iran. Conducted along one of the world’s most important maritime trade corridors, the engagement carries strategic implications that extend well beyond the immediate belligerents.

The incident occurred along a particularly consequential stretch of the Indian Ocean. Trade routes south of Sri Lanka carry significant volumes of global energy and container traffic linking the Middle East, Europe, and Asia. For China in particular, these sea lines of communication are central to sustaining economic growth and long-term energy security. The willingness and demonstrated effectiveness of the United States in employing force in this space will not have gone unnoticed.

The engagement also served as a practical reminder of the continued effectiveness of U.S. submarine capability. While few observers doubt the lethality or reach of American undersea forces, the sinking demonstrated in operational terms the difficulty surface vessels face in detecting and countering modern submarines. In this sense, the incident was not only about Iran. It also sent a broader signal about the willingness and ability of the United States to employ military force at range in defense of its interests. For some audiences this reinforces deterrence — for others it heightens concern about escalation along critical maritime trade routes.

Naval Warfare’s Enduring Logic

The sinking of the IRIS Dena was a stark reminder that naval warfare follows its own logic. Engagements can occur far from home waters, unfold with little warning, and carry consequences well beyond the immediate tactical exchange. In this case, a single submarine strike intersected with global trade flows, alliance dynamics, contested information environments, and the legal realities of conflict at sea.

For policymakers, the episode underscores the need to understand maritime warfare not as a peripheral concern but as a central feature of contemporary strategic competition. For naval commanders, it reinforces enduring truths about stealth, reach, and the unforgiving nature of combat at sea. More broadly, the incident signals that the United States remains willing to employ force along critical sea lines of communication when it judges such action lawful and strategically necessary. Maritime power is again shaping strategic outcomes in the Indo-Pacific.

Jennifer Parker is a former Royal Australian Navy warfare officer with more than two decades of service. She is an adjunct professor at the University of Western Australia Defence and Security Institute and a non-resident fellow at the Lowy Institute. She holds several tertiary qualifications, including degrees in law.

Image: United States Department of Defense via Wikimedia Commons