At least 87 people have been confirmed dead, with many more missing, after a US submarine hit the Iranian frigate IRIS Dena off the coast of Sri Lanka on Wednesday. While the Sri Lankan Navy rescued 32 Iranian sailors, an estimated 60 people on board remain missing.
The sinking of the IRIS Dena was the first time since World War II that a U.S. submarine had sunk another vessel with a torpedo.
IRIS Dena
The IRIS Dena, a 1,500-tonne Moudge-class frigate, was en route to Iran after taking part in a multilateral naval exercise in the Bay of Bengal.
Launched in 2015 and commissioned into the Iranian Navy in 2021, the IRIS Dena was a destroyer equipped with four Iranian “Bonyan 4” engines and a vertical launch system.
It was one of the warships that participated in the recently concluded Milan 2026 in Visakhapatnam in the Indian Ocean.
Milan 2026
Milan is the flagship of the Indian Navy, which hosts a multilateral naval exercise every two years. 13. The 13th edition of Milan was held from February 21 to February 25 under the theme “Friendship, Cooperation, Collaboration”.
According to the Indian Navy, 74 countries participated in Milan 2026, making it the largest and most inclusive to date, with Germany, the Philippines and the United Arab Emirates participating with military assets for the first time.
The exercises provide a platform for participating nations to learn and practice joint naval combat strategies and interoperability, the Navy said.
“Milan 2026, held under the theme ‘Camaraderie, Cooperation, Cooperation’, witnessed participation on an unprecedented scale involving 42 ships and submarines and 29 aircraft. These also included 18 ships from participating friendly foreign countries,” the Indian Navy said.
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The mega exercise was held over a week and culminated in a closing ceremony held aboard India’s indigenous aircraft carrier INS Vikrant off the coast of Visakhapatnam.
The exercise included both a port phase, which included a bilateral engagement and an international naval seminar, and a “sea phase”, which included high-intensity operational exercises focusing on advanced combat disciplines, including integrated air defense and anti-submarine warfare exercises, maritime interdiction operations, communications exercises and fly-over-board operations.
The goals of the exercise included enhancing the ability of the various navies to operate together as a cohesive force and focusing on collective responses to non-traditional threats such as piracy and drone swarms.
According to reports, this was the second time that IRIS Dena was part of Milan. An Iranian warship also participated in the 12th Milan in 2024.
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What the US said
Announcing the attack on the IRIS Dena on Wednesday, Chairman of the Joint Chiefs General Dan Caine said it was sunk using an MK-48 heavy torpedo.
Caine said the U.S. has sunk the Dena, over 20 Iranian vessels and an Iranian submarine since the U.S.-Israeli Operation Epic Fury began on Saturday.
“We have destroyed more than 20 Iranian naval vessels, in addition to an out-of-area frigate, one submarine, and at this point we have effectively neutralized Iran’s main naval presence on the ground,” he said.
Key things
- The 2026 Milan exercise showcased unprecedented international naval cooperation involving 74 countries and a variety of military assets.
- The sinking of the IRIS Dena highlights the evolving dynamics of naval warfare and the consequences of naval confrontations.
- The exercise was aimed at enhancing interoperability between navies to address collective security threats, reflecting a shift in global maritime strategy.
