On March 3, 2026, a U.S. Navy fast-attack submarine sank the Iranian frigate IRIS Dena off the southern coast of Sri Lanka. Defense Secretary Pete Hegseth claimed it was the first time a U.S. Navy sub had destroyed a vessel with a torpedo — in this case, a single Mark 48 – since the USS Torsk dropped two Japanese frigates on August 14, 1945. Ironically, the Dena was returning from a training exercise with 74 other countries, including the United States, when it was sunk. As what Hegseth called one of Iran’s “prize ships,” the Dena now joins at least two dozen other Iranian ships that have been sunk since the United States and Israel attacked Iran on February 28, 2026.

IRIS stands for the Islamic Republic of Iran Ship, the more commonly used designation for naval vessels of Iran. Sometimes, these ships are also referred to by the acronym IRINS, which stands for Islamic Republic of Iran Naval Ship. The primary function of the Dena was to patrol deep water. 

The Mowj (or Moudge) class of frigates of which the Dena was a part is based on the British Alvand light frigates. However, these frigates are referred to by the Iranian Navy as destroyers (despite there being a difference between the two classes). As one of Iran’s newest warships, the Dena was equipped with surface-to-air missiles, machine guns, torpedoes, anti-ship missiles, and a helicopter. However, whether it was armed at the time of its sinking is disputed between Iran and other countries.

Estimates from 2025 claim the Islamic Republic of Iran Navy (IRIN) has over 100 ships in its fleet and over 18,500 personnel. However, U.S. President Donald Trump has said the main objective of “Operation Epic Fury” is to destroy Iran’s navy, and the country’s naval forces have been hit hard since the military operation began.

Before the operation, the International Institute for Strategic Studies (IISS) annual assessment for 2026 estimated that IRIN had 18 submarines, 70 patrol/coastal combatant ships, a single mine warfare and countermeasures vessel, 23 amphibious vessels, and 17 logistics and support ships. The oldest is the IRIS Hamzeh, which was launched in 1936. This was originally the royal yacht of Reza Shah before becoming a 594-ton corvette. The IRIN’s largest ship was the massive IRINS Makran, which started life as a crude oil tanker and could carry as many as six helicopters. However, it too was recently destroyed by U.S. forces.

As many as 180 people were said to be aboard the IRIS Dena when it sank. Authorities at the hospital in Sri Lanka’s port city of Galle where casualties were brought reportedly received 87 bodies, while another 32 people were rescued. Approximately 60 others had still been unaccounted for. It remains to be seen how Iran’s fleet will sustain itself after this campaign is over.