Thousands of American troops could be in Iran’s direct line of fire if President Trump joins Israel in attacking Tehran’s nuclear program and military, as he said on Wednesday that he may or may not do.

Many would have only minutes to take cover from an incoming Iranian missile.

Experts expect that if Mr. Trump orders the American military to directly participate in Israel’s bombing campaign, Iran will quickly retaliate against U.S. troops stationed across the Middle East.

“The Americans should know that any U.S. military intervention will undoubtedly be accompanied by irreparable damage,” Iran’s supreme leader, Ayatollah Ali Khamenei, warned on Wednesday, according to state news media.

More than 40,000 U.S. active-duty troops and civilians are working for the Pentagon in the Middle East, and billions of dollars in weapons and military equipment are stored there. Over decades, both during and after war, the American military has fortified its defenses in the region, said Dana Stroul, the Pentagon’s top official for Middle East policy during the Biden administration.

The United States further strengthened those defenses, she said, after Hamas’s brutal attacks on Israel in October 2023, which set off a broader conflict between Israel and Iran’s regional allies.

“In some ways, the U.S. military has absolutely set the theater to respond to Iranian attacks, should the regime choose to turn its missiles or activate its militias against U.S. forces,” Ms. Stroul said on Wednesday.