WASHINGTON (AP) — The Trump administration and its allies in Congress presented a shifting new justification Monday for the U.S. attack on Iran, with House Speaker Mike Johnson suggesting that the White House believed Israel was determined to act on its own, leaving the president with a “very difficult decision."

The Republican was speaking late Monday after a classified briefing at the Capitol, the first for congressional leaders since the start of the war, a joint U.S.-Israel military campaign that killed Iran's Supreme Leader Ayatollah Ali Khamenei and has quickly spiraled into a widening Middle East conflict. Hundreds have died, including at least six U.S. military service personnel.

Johnson said the attack on Iran was a “defensive operation” because Israel was ready to act against Iran, “with or without American support.” He said President Donald Trump and his team determined that Iran would immediately retaliate against U.S. personnel and assets.

“The commander in chief has said this is going to be an operation that is short in duration,” Johnson said. “We certainly hope that’s true.”

The remarkable shift in the Trump administration's stated rationale comes as the hostilities deepen and widen across the region. The president himself estimated the war could drag on for weeks. The administration plans to seek supplemental funds from Congress to support the military effort, lawmakers said, in stark contrast to the president's America First campaign not to entangle the U.S. in actions abroad.

Secretary of State Marco Rubio said the “hardest hits are yet to come” as the U.S. is determined to continue attacking Iran for as long as it takes with an “even more punishing” next phase in the war.

Rubio described what was essentially a potentially ripple effect that he said posed an “imminent threat" to the U.S.

“We knew that there was going to be an Israeli action,” he said. “And we knew that if we didn’t preemptively go after them before they launched those attacks, we would suffer higher casualties.”

Rubio said that while the U.S. would like to see the Iranian people rise up and be rid of the regime, “that’s not the objective,” he said. “The objective of this mission is to make sure they don’t have these weapons that can threaten us and our allies in the region.”

Trump's shifting rationale sparks detractors

Rubio, Defense Secretary Pete Hegseth and other administration officials delivered the classified briefing as Congress weighs a war powers resolution that would restrain Trump’s ability to keep waging war without approval from the House and Senate.

Trump himself, speaking at the White House, laid out four objectives for the war, saying U.S. forces are out to destroy Iran’s missile capabilities, wipe out its naval capacity, stop the country from obtaining a nuclear weapon and ensure "that the Iranian regime cannot continue to arm, fund and direct terrorist armies outside of their borders.”

“This was our last, best chance to strike — what we’re doing right now — and eliminate the intolerable threats posed by this sick and sinister regime,” Trump said.

Trump met repeatedly with Israeli Prime Minister Benjamin Netanyahu as they sought to curb Iran's nuclear program, including last month at the White House.

Hegseth earlier Monday vowed this is not an “endless war,” even as he warned more U.S. casualties are likely in the weeks ahead.

But Sen. Mark Warner, the top Democrat on the Intelligence Committee, said: “There was no imminent threat to the United States of America by the Iranians. There was a threat to Israel.”

Warner said he has now heard four or five stated reasons for the attack. He demanded that Trump “come before Congress, and for that matter, the American people,” to make his case for war — and the exit plan.

Several Democrats delivered blistering speeches against the war. “Are we now such an enfeebled nation that Israel decides when we go to war?” said Sen. Jeff Merkley of Oregon, voice rising.

War powers as a check on presidential power

The moment is a defining one for Congress, which alone has the authority under the U.S. Constitution to declare war, and for the Republican president, who has consistently seized power during his second term with his own executive reach.

Trump took the nation to war at a particularly vulnerable time, as the Department of Homeland Security is operating without routine funds because of a standoff with Democrats over their demands to restrain his immigration enforcement operations. The potential wartime costs in terms of lives lost and dollars spent are dividing the parties, and potentially Americans themselves.

Unlike the run-up to the Iraq War in 2003, which included long debates in Congress in the aftermath of the Sept. 11, 2001, attacks, or the more recent U.S. military strikes on Venezuela that proved to be limited, the joint U.S.-Israel military attack on Iran, called Operation Epic Fury, is well underway, with no foreseeable end in sight.

“It’s worrisome,” Rep. Adam Smith, the top Democrat on the House Armed Services Committee, told The Associated Press.

Smith said of Trump: “He is not trying to making his case to the Congress or the American people. He unilaterally decided to do this.”

In fact, Congress has declared war just five times in the nation's history, most recently in 1941, to enter World War II a day after the Pearl Harbor attack. Over time, presidents of both major political parties have accumulated vast authority to engage in what are often more limited U.S. military strikes.

Johnson said tying Trump's hands right now would be “frightening" as he works to defeat the war powers resolution.

Even if Congress is able to pass the measure this week, the House and the Senate would be unlikely to tally the two-thirds majority needed to overcome a presidential veto.

Next steps for Iranian people uncertain

As the Trump administration encourages the Iranian people to rise up and choose new leaders, there did not appear to be widespread U.S. support for any effort at democracy- or nation-building.

“We would love to see this regime be replaced," Rubio said. “If there’s something we can do to help them down the road, we'd obviously be open to it. But that’s not the objective."

A top Trump ally, Sen. Lindsey Graham, R-S.C., said he never bought into the you-break-it-you-own-it concept in wartime.

“If there’s a threat to America, deal with it," he said over the weekend. "That doesn’t mean you own everything that follows.”

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Associated Press writer Kevin Freking contributed to this report.