"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.

Rubio, who was preparing to brief key US lawmakers, said that Iran had told field commanders to respond automatically against US forces if there was an 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.

Asked if the United States faced an imminent threat from Iran -- a key threshold in the United States as Congress constitutionally has the power to declare war -- Rubio again pointed to the Israeli plans.

"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 US forces first, "we would all be here answering questions about why we knew that and didn't act."

Representative Joaquin Castro, a Democrat, said that Rubio's comments indicated that Israel "put US forces in harm's way by insisting on attack on Iran."

"This is unacceptable of the President, and unacceptable of a country that calls itself our ally," Castro wrote on X.

Defense Secretary Pete Hegseth earlier said that it was Israel that carried out the strike Saturday in Tehran that killed Iran's supreme leader, Ayatollah Ali Khamenei, and other key officials, after intelligence emerged that they were meeting.

Rubio, however, said that President Donald Trump's administration believed in the need to strike Iran, regardless of how the timing was triggered.

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

Rubio said that the United States would like to see the overthrow of the Iran's clerical state but that that was not the goal.

"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."

On Saturday, Israel's defense minister Israel Katz described its missile attack against Iran as pre-emptive. 

Iran has said the US assault was unprovoked, occurring as Tehran and Washington were in negotiations on a nuclear accord.

President Trump has said the US faced an imminent threat from Iran that justified war, although he gave no specifics and some US lawmakers said he has shown no evidence to back that assessment.

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