US Submarine Sinks Iranian Frigate IRIS Dena, First Torpedo Kill Since WWII

Submarine Torpedo Sinks Iranian Frigate in Indian Ocean

The United States confirmed that a U.S. Navy submarine sank the Iranian Navy frigate IRIS Dena with a torpedo in international waters of the Indian Ocean on March 4, 2026. U.S. Defense Secretary Pete Hegseth stated the attack marked the first time since World War II that an American submarine has sunk a warship with a torpedo in combat. It is also the third confirmed instance globally of a surface warship being sunk by a submarine torpedo since 1945.

The strike occurred amid an expanding regional conflict involving Iran, Israel, and the United States. In the days surrounding the incident, military operations targeted Iranian missile infrastructure, while retaliatory actions were reported across the Gulf and parts of the Middle East. The broader conflict has resulted in more than 1,000 reported deaths in Iran and dozens in Lebanon, alongside disruptions to regional energy flows and air travel.

Location and Rescue Operations

IRIS Dena issued a distress call early March 4 before sinking approximately 40 kilometers south of Sri Lanka near Galle, outside Sri Lankan territorial waters. Sri Lankan naval and air units launched a rescue operation, deploying two navy ships and an aircraft.

The frigate carried an estimated crew of 180. Authorities reported that 32 critically wounded sailors were recovered and transported to Galle’s main hospital. Search and rescue operations continued under international maritime protocols for additional survivors.

Prior to the sinking, IRIS Dena had participated in the International Fleet Review in Visakhapatnam, India, in February 2026 and took part in the MILAN 2026 multinational naval exercise. The vessel docked in India on February 20 before continuing its deployment across the Indian Ocean.

Historical Context of Submarine Torpedo Engagements

Submarine attacks sinking surface warships have been rare since 1945. During World War II, U.S. submarines sank more than 1,300 Japanese vessels, but postwar naval warfare shifted toward airpower and anti-ship missiles.

The first confirmed postwar sinking occurred in 1971, when Pakistan’s PNS Hangor torpedoed the Indian frigate INS Khukri, killing 176 sailors. In 1982, the British submarine HMS Conqueror sank the Argentine cruiser ARA General Belgrano during the Falklands War, resulting in 323 fatalities and prompting Argentina to withdraw much of its surface fleet from open operations.

Throughout the Cold War and subsequent conflicts, U.S. submarines primarily conducted surveillance and launched cruise missiles rather than engaging surface ships with torpedoes.

Torpedo System and Capabilities

U.S. Navy attack submarines are equipped with the Mk-48 heavyweight torpedo, introduced in 1972 and continuously upgraded. The 533 mm-diameter weapon measures approximately 5.8 meters in length and weighs about 1,670 kilograms. It carries a 292-kilogram high-explosive warhead and can exceed speeds of 55 knots.

The Mk-48 uses wire guidance and active or passive sonar homing. Modern variants, including the Mod 7 CBASS, feature broadband sonar processing and improved shallow-water performance. The torpedo detonates beneath a vessel’s keel, creating a gas bubble that compromises structural integrity as it collapses.

IRIS Dena: Design and Armament

IRIS Dena was a Moudge-class frigate constructed at Bandar Abbas. Construction began in 2012; the vessel was launched in 2015 and commissioned in 2021. The ship measured approximately 95 meters in length, with a beam of 11.1 meters and a displacement of about 1,500 tonnes. Four diesel engines provided propulsion, enabling speeds up to 30 knots.

The frigate was equipped with an Asr three-dimensional radar system and electronic warfare capabilities. Armament included a 76 mm naval gun, a Fath 40 mm anti-aircraft cannon, 20 mm cannons, Noor or C-802 anti-ship missiles, Sayad 2 surface-to-air missiles, and two triple 324 mm torpedo launchers for anti-submarine warfare.