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 U.S. embassies in Jordan, Bahrain and Iraq have ordered non-emergency personnel and family members to leave the posts.
Previously, Bahrain was on an authorized departure for this nonessential staff and their relatives. This is the first order of this kind handed down in Jordan during the conflict.
-ABC News' Shannon Kingston
President Donald Trump took to social media late Monday night to tout U.S. munitions stockpiles.
"The United States Munitions Stockpiles have, at the medium and upper medium grade, never been higher or better - As was stated to me today, we have a virtually unlimited supply of these weapons," Trump wrote.
He said that the U.S. will "easily prevail" in this war and that the country is prepared for the operation to go on for "whatever it takes." Trump wrote that "Wars can be fought 'forever,' and very successfully, using just these supplies."
However, Trump did acknowledge that the stockpile of some of the highest-grade munitions is "not where we want it to be," blaming that on U.S. support for Ukraine.
"The United States is stocked, and ready to WIN, BIG!!!" Trump said at the end of the post.
-ABC News' Meghan Mistry
In a statement directed at Secretary of State Marco Rubio on Tuesday, Iranian Foreign Minister Abbas Araghchi said, "Mr. Rubio admitted what we all knew: U.S. has entered a war of choice on behalf of Israel. There was never any so-called Iranian 'threat.'"
Araghchi's comments came after Rubio on Monday gave a new explanation from the Trump administration about why the U.S. decided to launch military action against Iran now -- saying the threat from Iran was "imminent" because it knew it would retaliate against the U.S. if attacked, including by Israel.
He spoke to reporters before briefing the Gang of 8 congressional leaders on Capitol Hill.
"It was abundantly clear that if Iran came under attack by anyone -- the United States, Israel or anyone -- they were going to respond, and respond against the United States. The orders had been delegated down to the field commanders," Rubio said. "It was automatic."
"If we stood and waited for that attack to come first, before we hit them, we would suffer much higher casualties," Rubio went on. "We knew that there was going to be an Israeli action. We knew that that would precipitate an attack against American forces, and we knew that if we didn't preemptively go after them before they launched those attacks, we would suffer higher casualties."
Australia's Defense Minister Richard Marles said Tuesday that Iranian drones struck an Australian military facility in the United Arab Emirates on the first night of the conflict, but there were no injuries.
The drones struck Al Minhad Air Base, which is a logistics hub for Australia's Middle East operations near Dubai, Marles said during a news conference.
