US submarine sinks Iranian frigate with Mark 48 torpedo, first such kill since WWII

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Publication date: 2026-07-17 12:43:59

NAVAL BASE GUAM (Feb. 3, 2025) – Sailors assigned to the Virginia-class fast-attack submarine USS Minnesota (SSN 783) guide a Mark 48 ADCAP (Advanced Capability) torpedo during a weapons offload onboard Naval Base Guam, Feb. 3, 2025. Assigned to Commander, Submarine Squadron 15, based at Polaris Point, Naval Base Guam, Minnesota is one of five forward-deployed fast-attack submarines. Renowned for their unparalleled speed, endurance, stealth, and mobility, fast-attack submarines are the backbone of the Navy’s submarine force. Regarded as apex predators of the sea, Guam’s fast-attack submarines serve at the tip of the spear, helping to reaffirm the submarine force's forward-deployed presence in support of a free and open Indo-Pacific. (U.S. Navy photo by Mass Communication Specialist 1st Class Nikita Custer)

In March 2026, a U.S. submarine sank an Iranian frigate with a single Mark 48 torpedo, the first time an American submarine had torpedoed an enemy warship since 1945. The weapon that did it is more than half a century old, unglamorous, and largely ignored in an era obsessed with drones and hypersonic missiles. Yet a growing body of analysis, and now a rare combat kill, suggests this quiet 21-inch tube may be among the most important weapons in the U.S. Navy’s arsenal. Here is the case, and the case against it.

Meet the Mark 48 Torpedo

On March 4, 2026, in the Indian Ocean roughly 20 nautical miles south of Sri Lanka, a U.S. Navy fast-attack submarine fired on the Iranian frigate IRIS Dena. The Pentagon later released infrared periscope footage showing the result: a massive detonation at the frigate’s stern, which lifted the ship, broke its hull, and sent it under. Defense Secretary Pete Hegseth confirmed the strike alongside Joint Chiefs Chairman Gen. Dan Caine, describing it as a “quiet death” and the first sinking of an enemy ship by a torpedo since World War II. Caine specified that the vessel had been destroyed by a single Mark 48 torpedo.

It was a grave event as well as a historic one. The Dena, a Moudge-class frigate, was returning from an international naval review in India and was transiting international waters when it was hit; of roughly 180 sailors aboard, only 32 were rescued, and at least 80 were killed. The circumstances, a strike far from the war’s center on a ship returning from a diplomatic gathering, drew sharp condemnation from Tehran. Whatever one concludes about those circumstances, the engagement demonstrated something concrete about the weapon involved, a weapon that had not sunk a ship in anger in eight decades and that most observers of modern warfare rarely think about at all.

The Weapon

The Mark 48 has been the United States Navy’s heavyweight submarine torpedo since 1972. In its current Advanced Capability form, it is the sole submarine-launched torpedo carried by every class of American submarine, from the Virginia- and Los Angeles-class attack boats to the Ohio-class ballistic-missile submarines that carry it purely for self-defense. It is a 21-inch-diameter, nineteen-foot weapon that can be steered from the launching submarine through a thin guidance wire or left to hunt on its own using active or passive acoustic homing, and if it misses on its first pass, it can circle and attack again.

What makes it so lethal is how it kills. Rather than striking a ship’s side like a missile, the Mark 48’s roughly 650-pound warhead is designed to detonate in the water directly beneath the hull. That explosion creates a gas bubble that lifts the ship’s midsection and then collapses, snapping the keel, the structural spine of the vessel, and breaking the ship apart. A missile that hits above the waterline can leave a warship damaged but afloat; a torpedo detonating under the keel can break a ship’s back and sink it outright, which is precisely what the IRIS Dena footage showed. Just as important, there is no reliable defense against it. Warships carry layered systems to shoot down incoming missiles and aircraft, but a heavyweight torpedo running deep beneath the surface is extraordinarily difficult to stop.

Why the Undersea Domain Decides

The argument that this makes the Mark 48 a weapon of the first rank does not rest on one strike against a lightly defended frigate. It rests on where the future of naval warfare is heading, and on what happens when American planners game out the fight everyone worries about most: a Chinese invasion of Taiwan.

When the Center for Strategic and International Studies ran exactly that scenario, playing it twenty-four times across a range of assumptions, one finding stood out: a mon