U.S. Submarine Sinks Iranian Frigate Off Sri Lanka, Expanding War’s Reach Into the Indian Ocean

Sri Lankan sailors searching the Indian Ocean at dawn this week found only an oil slick, splintered life rafts and bodies drifting in orange vests. The Iranian warship that had sent a frantic distress call hours earlier was gone—snapped in two and on the seabed 40 nautical miles south of Galle.

What happened in the dark before that sunrise marks a watershed in the expanding war between the United States, Israel and Iran—and a jolt for India and Sri Lanka, whose waters have suddenly become a stage for high-end naval combat.

The Pentagon has confirmed that a U.S. Navy fast-attack submarine fired a heavyweight torpedo that sank the Iranian frigate IRIS Dena in international waters off Sri Lanka in the early hours of March 4 local time. Sri Lankan officials say about 180 sailors were believed to be aboard. Thirty-two were rescued alive; 87 bodies have been recovered. Dozens more are missing and presumed dead.

“For the first time since 1945, a United States Navy fast-attack submarine has sunk an enemy combatant ship using a single Mk-48 torpedo,” a senior Pentagon spokesman said at a briefing in Washington. Defense Secretary Pete Hegseth later called the Dena Iran’s “prize ship” and said it was “one of the few instances of a submarine sinking a ship since World War II.”

The strike is the first publicly acknowledged case of a U.S. submarine torpedoing an enemy warship since the end of World War II and the first admitted submarine sinking of a surface combatant anywhere since the British attack on the Argentine cruiser General Belgrano in 1982.

A warship vanishes off Sri Lanka

Sri Lanka’s navy says it received a mayday call from the Dena around midnight GMT on March 4, reporting that the ship had been hit and was taking on water south of the island’s southern tip.

“When our vessels reached the given location there was no ship visible, only oil patches and life rafts,” navy spokesman Commander Buddhika Sampath told reporters in Colombo. “Survivors were found floating in life jackets in the water.”

Foreign Minister Vijitha Herath told Parliament the Iranian frigate had roughly 180 people on board when it went down. Survivors and the dead were brought to the port city of Galle and transferred to hospitals, including the teaching hospital at Karapitiya, where staff set up makeshift facilities to handle the influx.

Photographs from Galle showed Sri Lankan personnel unloading bodies in black bags from trucks under police and naval guard. Authorities said many of the dead had suffered blast injuries consistent with a massive underwater explosion.

The U.S. Defense Department later released black-and-white periscope video from the attacking submarine. The footage shows a sleek frigate—identified by the Pentagon as IRIS Dena—cutting through calm seas with sailors visible on deck. An underwater flash blooms beneath the hull, followed by a huge column of water and debris. The ship’s spine appears to buckle. Within minutes, only empty sea remains.

Pentagon officials said the weapon was a single Mk-48 heavyweight torpedo, the Navy’s standard submarine-launched weapon against ships and submarines. Modern variants are designed to detonate under a target’s keel, using the blast to break a hull rather than merely punch a hole in its side.

The United States has not publicly identified the submarine involved or its class, citing operational security.

Iran’s ‘prize ship’

The Dena was one of the newest and most modern ships in the Islamic Republic of Iran Navy. Commissioned in 2021, the roughly 1,500- to 1,800-ton Moudge-class frigate was equipped with a 76-millimeter main gun, anti-ship cruise missiles believed to be from the Noor family, Sayyad-2 surface-to-air missiles for air defense, lightweight torpedoes and a helicopter deck.

Iranian officials had showcased the ship in recent years as evidence that the country could build and operate modern surface combatants despite sanctions. The U.S. Treasury Department in 2023 sanctioned the Dena and related entities in connection with Iranian drone transfers to Russia.

Just days before it was sunk, the Dena had been a guest of the Indian Navy. It took part in the multilateral exercise MILAN 2026 and the International Fleet Review off Visakhapatnam on India’s east coast, appearing alongside warships from dozens of countries.

Indian officials have not said when the Dena left Visakhapatnam, but tracking agencies and regional media reported the frigate transited southwest across the Bay of Bengal and rounded Sri Lanka, apparently heading for home, before it was struck.

The timing and location underscore the expanding geographic scope of the conflict. The sinking took place far from the Persian Gulf and Strait of Hormuz, where Iranian and U.S. forces have clashed for decades, and in waters often described by Indian strategists as India’s “maritime backyard.”

India and Sri Lanka caught in the middle

New Delhi has reacted cautiously in public. India maintains long-standing political and economic ties with Iran, including cooperation on the Chabahar port project, even as it has deepened strategic and defense cooperation with the United States.

Indian media and security analysts have been less restrained. Commentators have noted that a warship invited to Indian waters for a ceremonial fleet review was destroyed days later just off neighboring Sri Lanka, and have questioned what the episode says about India’s ability to manage military tensions in its wider maritime sphere.

“The Iran–U.S.–Israel war has come right to India’s doorstep,” one New Delhi-based defense analyst said in a televised discussion, calling the incident “a dramatic widening of the war into India’s strategic waters.”

Sri Lanka, still recovering from economic turmoil and eager to avoid being drawn into great-power rivalries, has emphasized its humanitarian role.

Herath told lawmakers the attack occurred outside Sri Lanka’s 12-nautical-mile territorial sea but within its designated search-and-rescue region. Under the United Nations Convention on the Law of the Sea, coastal states must render assistance to vessels in distress regardless of flag.

“Our navy and air force responded promptly to the distress call, in keeping with our international obligations,” he said. Sri Lankan officials have not publicly criticized either Washington or Tehran over the incident.

Legal gray zones at sea

The United States frames the sinking as a lawful act of war in an ongoing armed conflict with Iran. U.S. and Israeli forces have carried out widespread strikes on Iranian government, military and paramilitary targets since late February, including attacks that killed Supreme Leader Ayatollah Ali Khamenei and destroyed other high-value naval assets such as the Iranian drone carrier Shahid Bagheri and the forward base ship Makran.

“We are systematically degrading Iran’s ability to project power at sea,” Hegseth said, adding that eliminating Iranian naval threats was a key objective of the campaign.

Washington has not, however, publicly detailed what specific threat the Dena posed at the moment it was targeted. Open-source accounts place the ship on a return voyage from exercises in India. There is no public indication it was engaged in immediate hostile action near Sri Lanka when it was struck.

Under international law, belligerents in an armed conflict may attack the armed forces and military objectives of the other side, including warships, even on the high seas and in exclusive economic zones. Formal declarations of war, once common, are not required for a conflict to exist.

Still, legal scholars and advocacy groups are likely to scrutinize questions of necessity and proportionality. Some online commentators and experts have asked whether sinking a warship transiting after a friendly port visit, far from any active battlefront, was militarily necessary or risked unduly expanding the war zone.

The attack also raises issues for neutral coastal states. While countries do not control military operations in their exclusive economic zones, large-scale combat in these waters can affect their security and economic interests. To date, neither India nor Sri Lanka has indicated that it will lodge a formal diplomatic protest.

A signal beneath the waves

Beyond its immediate tactical impact, the Dena’s destruction is widely seen as a message.

Submarine operations are typically hidden, and navies rarely release detailed footage of torpedo strikes. The Pentagon’s decision to publish periscope video of the attack suggests officials wanted to highlight U.S. undersea capabilities not only to Iran, but to other potential adversaries and to domestic audiences.

Analysts say the visuals—a modern warship torn apart in seconds—serve as a graphic demonstration of what a single submarine can do in contested waters, and a warning to states contemplating challenging U.S. or allied navies from the Gulf to the Western Pacific.

They also underline the steep technological asymmetry between Iran’s conventional fleet and the U.S. Navy. U.S. Central Command officials have said at least 17 Iranian naval vessels have been destroyed in the current conflict, a significant portion of Tehran’s blue-water surface assets.

For Sri Lanka and India, the sight of an advanced frigate vanishing just off their coasts is a different kind of signal. It shows that the northern Indian Ocean—a corridor for much of the world’s oil and container traffic—is now an active theater in a war that began more than 3,000 kilometers away.

Shipping companies and insurers are watching closely. Even though the Dena was a military target, not a merchant ship, war-risk underwriters often adjust premiums for entire regions when state-on-state attacks occur along key sea lanes. Any Iranian attempt to retaliate at sea—whether in the Arabian Sea, the Gulf of Aden or the Red Sea—could further disrupt trade and drive up costs.

As Sri Lankan patrol craft continue to comb the waters south of Galle for bodies and debris, little remains visible of the Dena’s brief final voyage from an Indian port. But the consequences of its loss are likely to linger: in Tehran’s diminished naval reach, in New Delhi’s strategic calculations and in a stretch of the Indian Ocean that now bears the imprint of a submerged American warship and a single torpedo track through the dark.