The United States launched its attack on Iran after determining that Israel was preparing its own strike and concluding that Tehran would respond by targeting American forces, Secretary of State Marco Rubio said Monday, framing the operation as a preemptive move designed to reduce U.S. casualties.

Speaking to reporters ahead of a classified briefing for senior lawmakers, Rubio said U.S. officials were aware in advance that Israel intended to act and assessed that Iran would quickly retaliate against American personnel in the region.

“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,” Rubio told reporters.

According to Rubio, intelligence indicated that Iranian leadership had instructed commanders in the field to automatically strike U.S. forces if Iran came under attack.

“If we stood and waited for that attack to come first before we hit them, we would suffer much higher casualties. And so the president made the very wise decision” to hit alongside Israel, Rubio said.

When questioned about whether the United States faced an imminent threat from Iran — a key legal consideration given Congress’s constitutional authority to declare war — Rubio again pointed to the anticipated Israeli action and Iran’s expected response.

“There absolutely was an imminent threat, and the imminent threat was that we knew that if Iran was attacked — and we believed they would be attacked — that they would immediately come after us,” Rubio said.

“We were not going to sit there and absorb a blow,” Rubio said, adding that if Iran hit U.S. forces first, “we would all be here answering questions about why we knew that and didn’t act.”

Earlier, War Secretary Pete Hegseth said that Israel carried out Saturday’s strike in Tehran that killed Iran’s supreme leader, Ayatollah Ali Khamenei, along with other senior officials, after intelligence showed they were gathered for a meeting.

Rubio emphasized, however, that President Donald Trump’s administration viewed military action against Iran as necessary independent of the precise timing or sequence of events.

“No matter what, ultimately this operation needed to happen,” Rubio said.

While expressing hope for political change inside Iran, Rubio made clear that regime change was not the stated objective of the mission.

“We hope that the Iranian people can overthrow this government and establish a new future for that country. We would love for that to be possible,” Rubio said.

“But the objective of this mission is the destruction of their ballistic missile capabilities and of their naval capabilities.”