On March 3, 2026, a U.S. Navy fast-attack submarine sank the Iranian frigate IRIS Dena near the southern coast of Sri Lanka. U.S. Defense Secretary Pete Hegseth reported this incident as the first occurrence of a U.S. submarine destroying a vessel with a torpedo—specifically a Mark 48—since the sinking of two Japanese frigates by the USS Torsk on August 14, 1945. Notably, the Dena had just completed a training exercise alongside 74 other nations, including the United States, when it was sunk. Dubbed a “prized ship” by Hegseth, the Dena joins the ranks of over two dozen Iranian vessels lost since the U.S. and Israel initiated strikes against Iran on February 28, 2026.
The term IRIS denotes Islamic Republic of Iran Ship, a title generally attributed to Iran’s naval vessels. Alternatively, the acronym IRINS, meaning Islamic Republic of Iran Naval Ship, is also commonly used. The primary role of the Dena was to conduct operations in deep waters.
Part of the Mowj (or Moudge) class of frigates, the Dena was modeled after British Alvand light frigates, although the Iranian Navy designates these as destroyers despite the technical distinctions. As one of the latest additions to Iran’s warship fleet, the Dena was outfitted with surface-to-air weapons, machine guns, torpedoes, anti-ship missiles, and a helicopter. Nonetheless, there is debate surrounding whether it was armed at the time of its sinking, with varying accounts from Iran and its adversaries.
Challenges Facing the Islamic Republic of Iran Navy
As per 2025 estimates, the Islamic Republic of Iran Navy (IRIN) reportedly consists of over 100 ships and more than 18,500 personnel. However, U.S. President Donald Trump has indicated that the focus of “Operation Epic Fury” is to incapacitate Iran’s naval capabilities, leading to significant losses for the Iranian fleet since the onset of the military campaign.
Prior to the operation, the International Institute for Strategic Studies (IISS) assessed that in 2026, IRIN’s fleet included 18 submarines, 70 patrol and coastal combatants, one mine warfare vessel, 23 amphibious craft, and 17 logistics and support ships. The IRIS Hamzeh, launched in 1936 and formerly the royal yacht of Reza Shah, is the fleet’s oldest asset, now retrofitted into a 594-ton corvette. The IRINS Makran, the largest ship in the fleet, transitioned from an oil tanker to a formidable vessel capable of accommodating six helicopters but was also reportedly destroyed by U.S. forces recently.
Onboard the IRIS Dena at the time of its sinking were approximately 180 crew members. When casualties were reported at a hospital in Galle, Sri Lanka, authorities received 87 bodies, while 32 individuals were rescued. Approximately 60 crew members remain unaccounted for. The future viability of Iran’s military fleet following this conflict is uncertain.
