A multi-million-dollar U.S. Navy torpedo detonated underneath an Iranian warship in a nighttime submarine strike off Sri Lanka’s southern coast — an attack, War Secretary Pete Hegseth said Wednesday in a Pentagon update, was the first of its kind since World War II.

The weapon, identified as a Mark 48 Advanced Capability (ADCAP) torpedo, underscored the scale of force used, and signaled to Tehran that "the gloves really are off," according to a former U.S. submarine commander.

"The Mark 48 is one of the most lethal anti-ship weapons in the U.S. inventory," American submarine sank an Iranian warship that thought it was safe in international waters," Hegseth told reporters at the Wednesday briefing.

Hesgeth described the strike as "a quiet death," adding that it marked the first sinking of an enemy ship by torpedo since World War II.

"The U.S. Navy submarines are very highly mobile, very, very quiet, and our crews are extremely well-trained," Shugart explained. "This was not a challenge for a U.S. Navy submarine to fire a torpedo."

"To hunt down and sink an Iranian ship like that is not — that's not a challenging task for a U.S. nuclear-powered submarine," he said.

The targeted vessel, identified as the IRIS Dena, was the newest frigate in Iran’s naval fleet and was equipped with surface-to-air missiles, anti-ship missiles, torpedo launchers and other heavy weaponry.

According to Sri Lanka’s Foreign Affairs Minister Vijitha Herath, the country’s coast guard received a distress call at 5:08 a.m. local time Wednesday from the Iranian vessel reporting an explosion.

"I'm not sure Iran has any operational submarines anymore, but if they were operational, their biggest submarines would be at least 20 or 30 years old," Shugart said.

"They would be ex-Russian diesel-electric submarines, so they're not nuclear-powered like the U.S. ones, with satellite communications and unlimited mobility."

"The U.S. submarines can operate at high speed for as long as they want with unlimited endurance, other than the food on board. They carry the most advanced weapons, the most advanced sensors."

"This strike sent a message that if there are any Iranian warships left or any Iranian government-owned ships, they should expect no mercy," he added.