US submarine sinks first warship with torpedo in combat since World War II with attack on Iranian frigate IRIS Dena.

The US confirms that a US Navy submarine sank the Iranian Navy frigate IRIS Dena with a torpedo in the Indian Ocean on March 4, 2026, marking the first time a US submarine has sunk a warship with a torpedo in combat since World War II.

US Defense Secretary Pete Hegseth confirmed that a US Navy submarine sank the Iranian frigate IRIS Dena with a torpedo in international waters of the Indian Ocean on March 4, 2026. It is the first time a US submarine has sunk a warship with a torpedo in combat since World War II, and the sinking represents the third confirmed case of a surface warship being sunk by a submarine torpedo attack since the end of World War II, following the INS Khukri in 1971 and the ARA General Belgrano in 1982.
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The confirmation by Pete Hegseth of the US submarine attack on the Iranian frigate IRIS Dena marks the first time a US submarine has sunk a warship with a torpedo in combat since World War II. (Picture source: US DoW and Iranian MoD)

On March 4, 2026, the United States confirmed that a submarine torpedo attack sank the Iranian frigate IRIS Dena in the Indian Ocean during the expanding conflict involving Iran, Israel, and the United States. U.S. Defense Secretary Pete Hegseth announced that an American submarine struck an Iranian warship operating in international waters with a torpedo, resulting in the first sinking of an enemy vessel with such a US Navy attack since the end of WWII. The strike occurred during a period of intensified military operations targeting Iranian military infrastructure and missile capabilities across the region. Iran conducted retaliatory actions across the Gulf and the broader Middle East, while strikes also hit locations in Lebanon linked to Hezbollah.

The conflict caused more than 1,000 deaths in Iran and dozens in Lebanon while disrupting energy supplies and international travel across the region. In the maritime domain, the sinking of the Iranian warship in the Indian Ocean represented a rare instance of a submarine torpedo attack against a surface combatant during modern naval warfare. Submarine attacks that sink warships have been extremely uncommon since the end of the Second World War, reflecting changes in naval warfare and the limited number of fleet engagements between naval powers. During World War II, United States submarines alone sank more than 1,300 Japanese ships in the Pacific theatre, making submarine warfare a major component of maritime combat. After 1945, the frequency of such engagements declined sharply as naval forces increasingly relied on aircraft and anti-ship missiles rather than torpedoes.

One of the first confirmed postwar submarine sinkings occurred on December 9, 1971, during the Indo-Pakistani War, when the Pakistani submarine PNS Hangor fired homing torpedoes at the Indian Navy frigate INS Khukri in the Arabian Sea. The ship sank rapidly after the attack, and 176 sailors were killed. The incident marked the first confirmed case of a warship sunk by a submarine since the end of the Second World War. Another major submarine torpedo attack against a surface warship occurred on May 2, 1982, during the Falklands War. The British nuclear submarine HMS Conqueror fired three Mk-8 torpedoes at the Argentine cruiser ARA General Belgrano while the ship was operating in the South Atlantic.

Two torpedoes struck the cruiser and caused severe structural damage, leading to the sinking of the vessel. A total of 323 Argentine sailors were killed in the incident, making it one of the deadliest naval losses of the conflict. The attack also had immediate operational consequences as the Argentine Navy withdrew most of its surface fleet from open operations for the remainder of the war. These two cases illustrate how submarine attacks against warships became rare events in the decades following 1945. Modern naval combat shifted toward long-range missiles, aircraft strikes, and electronic surveillance. For the US Navy specifically, there was no confirmed case of an American submarine sinking an enemy military vessel in combat between 1945 and the early 2020s, until the sinking of the Iranian Navy frigate IRIS Dena in the Indian Ocean on March 4, 2026.

During the Cold War, U.S. attack submarines frequently tracked Soviet submarines and surface groups in the Atlantic and Pacific, but rules of engagement prevented torpedo attacks unless open war began. In later conflicts such as the Vietnam War, the Gulf War, and operations in Iraq, Afghanistan, and Libya, submarines were employed mainly for intelligence collection and land attack missions. Nuclear-powered attack submarines launched cruise missiles such as the Tomahawk against ground targets while also conducting surveillance patrols and special forces insertion missions. These missions reflected the broader shift in submarine operations toward strategic deterrence and long-range strike roles rather than direct torpedo engagements with surface ships.

The principal torpedo used by U.S. Navy submarines is the Mk-48 heavyweight torpedo, which entered operational service in 1972 and has undergone several upgrades since that time. The torpedo has a diameter of 533 millimetres, a length of about 5.8 metres, and a weight of about 1,670 kilograms. It carries a high-explosive warhead weighing about 292 kilograms and can reach speeds exceeding 55 knots depending on its operational profile. The weapon is capable of engaging both submarines and surface ships using a combination of wire guidance and active or passive sonar homing. Modern variants such as the Mk-48 Mod 7 CBASS incorporate broadband sonar processing and improved performance in shallow water environments.

The torpedo detonates beneath the keel of a ship, producing a gas bubble that lifts the hull and causes structural failure when the bubble collapses. The warship that sank near Sri Lanka was identified as the Iranian Navy frigate IRIS Dena, which issued a distress call early on March 4, 2026, before going down in the Indian Ocean. The vessel sank about 40 kilometres south of Sri Lanka near the coastal city of Galle, outside the island’s territorial waters. Sri Lankan naval and air units launched a rescue operation after receiving the distress signal and deployed two navy ships and an aircraft to the location. The frigate carried a crew of about 180 sailors at the time of the incident. Rescue teams recovered 32 critically wounded sailors from the water and transported them to the main hospital in Galle for treatment.

Search operations continued in the surrounding waters for the remaining members of the crew while the rescue effort proceeded under international maritime obligations. Before the sinking, IRIS Dena had taken part in international naval activities in India during February 2026. The frigate participated in the International Fleet Review held in the Indian port city of Visakhapatnam, which gathered naval forces from multiple countries for maritime ceremonies and fleet events. After the fleet review ceremony on February 18, the ship docked in Visakhapatnam on February 20 as part of its visit. The vessel also joined the MILAN 2026 multinational naval exercise, which combined harbour activities with a sea phase involving coordinated maritime drills between participating navies.

Iran’s naval commander Commodore Shahram Irani attended the events and met India’s Chief of the Naval Staff Admiral Dinesh K Tripathi during the visit. These activities took place shortly before the frigate departed India and continued its voyage across the Indian Ocean. IRIS Dena belonged to the Moudge class of domestically produced Iranian frigates developed for operations in the Persian Gulf, the Arabian Sea, and the Indian Ocean. The ship was constructed at naval facilities in Bandar Abbas, where construction began in 2012 before the vessel was launched in 2015 and commissioned into service in 2021.

The frigate measured about 95 metres in length with a beam of about 11.1 metres and a draught of about 3.25 metres, with a displacement reaching about 1,500 tonnes. Propulsion consisted of four diesel engines rated at 5,000 horsepower each, supported by diesel generators, enabling a maximum speed of about 30 knots. The vessel carried an Asr three-dimensional radar system and electronic warfare equipment for target detection and tracking. 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.

Written by Jérôme Brahy

Jérôme Brahy is a defense analyst and documentalist at Army Recognition. He specializes in naval modernization, aviation, drones, armored vehicles, and artillery, with a focus on strategic developments in the United States, China, Ukraine, Russia, Türkiye, and Belgium. His analyses go beyond the facts, providing context, identifying key actors, and explaining why defense news matters on a global scale.