Sgt. Benjamin N. Pennington, 26, was killed in Saudi Arabia.
President Donald Trump announced "major combat operations" against Iran on Feb. 28, with massive joint U.S.-Israel strikes attack targeting military and government sites, officials said.
Iranian state television confirmed that Ayatollah Ali Khamenei was among those killed in Tehran on the first day of strikes. His son Mojtaba Khamenei was chosen on Sunday to succeed him.
Iran is responding to the operation with missile and drone attacks targeting Israel, regional U.S. bases and multiple Gulf nations. Israel is also intensifying its long-running strike campaign against the Iran-backed Hezbollah militia in Lebanon.
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The secretary of Iran's Supreme National Security Council, Ali Larijani, said that Iran is prepared for a long war.
"Iran, unlike the United States, has prepared itself for a long war," Larijani wrote in a post on X on Monday. He added that Iranian armed forces "have not engaged in any attacks except in defense."
Earlier on Monday, Larijani said the Islamic Republic will not negotiate with the U.S.
Defense Secretary Pete Hegseth said "the mission of Operation Epic Fury is laser-focused: destroy Iranian offensive missiles, destroy Iranian missile production, destroy their Navy and other security infrastructure, and they will never have nuclear weapons."
Gen. Dan Caine, the chairman of the Joint Chiefs of Staff, said there were more than 1,000 targets in the first 24 hours.
"With every passing day, our capabilities get stronger and Iran's get weaker," he said. "We set the terms of this war from start to finish. Our ambitions are not utopian -- they are realistic, scoped to our interests and the defense of our people and our allies."
"This is not Iraq, this is not endless," Hegseth said. "... (Trump) called the last 20 years of nation-building wars dumb, and he's right. This is the opposite. This operation is a clear, devastating, decisive mission: destroy the missile threat, destroy the Navy, no nukes."
Defense Secretary Pete Hegseth called the ongoing operation "our retribution" for Iran's yearlong role in sponsoring terrorism, including roadside bombs used against troops during the Iraq war.
Iran's "war on Americans has become our retribution against their Ayatollah and his death cult," he said. "It took the 47th president, a fighter who always puts America first, to finally draw the line after 47 years of Iranian belligerence."
Defense Secretary Pete Hegseth said at a news conference Monday, "We didn't start this war, but under President Trump, we are finishing it."
Hegseth said Iran was building "powerful missiles and drones to create a conventional shield for their nuclear blackmail ambitions," with "our bases, our people, our allies all in their crosshairs."
He said Iran "tried to lie their way to a nuclear bomb."
He said the U.S. "bent over backwards for diplomacy" but "Tehran was not negotiating," and stalled to rebuild their stockpiles.
