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Iranian Frigate Sunk by U.S. Submarine Off Coast of Sri Lanka

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IRIS Dena was one of Iran’s newest combat ships, and had 180 personnel on board when the submarine attacked.

Reports of the Dena being in distress first emerged overnight between Mar. 3 and Mar. 4, 2026, from Sri Lankan sources. The ship was operating outside of territorial waters near the southwestern city of Galle, likely remaining distant from their home in an ultimately unsuccessful attempt to avoid becoming a victim of the U.S.-Israeli strikes against Iran. 

IRIS Dena was one of Iran’s newest combat ships, and had 180 personnel on board when the submarine attacked.Uncertainties of Conflict 

Dena’s sister vessels, Jamaran and Sahand, had already been completely destroyed in the earlier days of Operation Epic Fury, alongside Iran’s recently unveiled drone carrier IRIS Shahid Bagheri. Only one ship of the Moudge class, a 1500 ton light frigate armed with guns and a small number of missiles, apparently now remains in active service as a frigate.  

By the time Sri Lankan search and rescue forces, which included ships and aircraft, reached the scene the frigate was already completely submerged. Multiple figures are being reported as to how many crew members have been rescued, ranging from 32 to 79, but it is clear that will have been many fatalities among the 180 said to have been on board. 

Sir Lankan sources disputed the idea that a U.S. submarine had been involved in the sinking, but U.S. officials speaking anonymously to Reuters later confirmed that the U.S. was responsible. In a press conference, it was then officially confirmed by U.S. Secretary of Defense Pete Hegseth that a U.S. Navy submarine destroyed the vessel with a torpedo. 

This news marks only the second known time in history that a nuclear submarine had sunk another vessel in combat. The only previous example was during the Falklands War, where Royal Navy submarine HMS Conqueror destroyed the Argentinian cruiser ARA General Belgrano with a salvo of Mark VIII straight-firing torpedoes. 

U.S. Chairman of the Joint Chiefs of Staff General Dan Caine confirmed to reporters that the ship was destroyed by a single Mark 48 torpedo fired from a U.S. Navy fast attack submarine, with video footage taken by the submarine showing the ship’s hull essentially broken in half by the explosive force of the weapon. 

Uncertainties of Conflict 

A surprise attack by a submarine is a timely reminder that there are many things about the ongoing conflict that, as civilian observers, we simply cannot be absolutely certain on. It has been said on a number of occasions by U.S. officials in recent days that alongside all of the announced assets involved in Operation Epic Fury, U.S. forces are also deploying ‘classified effects’ against Iran. Some of these are almost certainly electronic and cyber warfare based, though the use of still-classified munitions and even aircraft is not only far from impossible – it’s likely. 

The War Zone reported on Feb. 28 on the appearance of a new variant of the Tomahawk missile, which sported an unusual glossy black finish rather than the dull grey usually seen. This is theorized to be a low observable (LO) enhancing feature, potentially part of the upgrade package delivered as the Maritime Strike Tomahawk. It appears essentially identical to the coating seen on the AGM-158C Long Range Anti-ship Missile (LRASM). 

Additionally, footage of cruise missiles in flight have shown designs with forward swept wings. The appearance of the missile, otherwise, is similar to a Tomahawk – if it is indeed a Tomahawk this would be the first time one has been seen with this modification. Forward swept wings would contribute to lowering the missile’s radar cross-section, but additionally could offer improved maneuverability which is an additional measure for increased survivability. 

It’s almost certain as well that the mysterious ‘RQ-180’ long range surveillance drone has also been active in this conflict, though to date we have only seen footage of a smaller Israeli flying wing drone. This has been claimed as the secretive ‘RA-01’, but we assess it to have actually been a different type. 

There have now been thousands of strikes against Iranian targets, and although civilian open-source intelligence analysts have been working hard to verify and geolocate images and footage that has made it into the public domain, social media is still littered with examples of old footage being passed off as current or even AI-generated content being claimed as legitimate. While being mindful of sources is good advice at any time, during events like this the need to verify and trace the accuracy of reporting is incredibly important. 


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