In a legacy-defining gamble, President Barack Obama announced Saturday that he has decided to launch military strikes against Syria — but wants the Congress to authorize them.

“In a world with many dangers, this menace must be confronted,” Obama declared in the Rose Garden 10 days after Bashar Assad’s forces allegedly massacred 1,400 civilians with chemical weapons.

“After careful deliberation, I have decided that the United States should take military action against Syrian regime targets,” he said, describing himself as “prepared to give that order.”

The president’s hastily arranged remarks — demonstrators protesting outside the White House gates could be heard from the West Wing only minutes before he spoke — sucked the urgency out of what had looked like a imminent military strike.

Instead, cruise missile-carrying warships off Syria’s coast will have to wait until the week of Sept. 9. That’s when Congress returns from a month-long vacation to take up a measure, drafted by the White House, giving Obama the green-light.

“I’m the president of the world’s oldest constitutional democracy," Obama said. "I will seek authorization for the use of force from the American people's representatives in Congress."

The president ignored a reporter who shouted the obvious question: What happens if Congress says no?

But senior administration officials briefing reporters at the White House later said that Obama still believes he has the legal authority to act without congressional support — meaning that a “no” vote would not necessarily handcuff his foreign policy. And they disputed that Obama risked setting a precedent that could limit the power of future occupants of the Oval Office.

The same officials also sidestepped repeated questions about what happens if Assad responds by stepping up chemical attacks against rebels looking to oust him.

The president himself said there was no sell-by-date for action. “Our capacity to execute this mission is not time-sensitive; it will be effective tomorrow, or next week, or one month from now,” he said.

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Obama’s decision came amid public opinion polls showing four out of five Americans wanted the president to seek lawmakers’ approval, and with more than 100 congressional signatures on a pair of letters delivering the same message.

Obama has acknowledged repeatedly that Americans are “war-weary” after a decade of conflict — and worried about standing on the threshold of another escalating entanglement in the Middle East.

“This would not be an open-ended intervention, we would not put boots on the ground,” he promised Saturday. “Instead, our action would be designed to be limited in duration and scope.”

The president said he had spoken by telephone with Republican House Speaker John Boehner and Democratic Senate Majority Leader Harry Reid and that they agreed with the timetable.

It also followed a series of diplomatic setbacks: Russian opposition blocked a path through the United Nations Security Council, and Britain’s parliament shocked the world Thursday by voting against military action. France signed on, but its parliament planned to debate the issue next week.

Denied both clear international legal legitimacy and a robust “coalition of the willing,” facing clear public resistance as well as a surprisingly assertive Congress, and trapped by his own declaration that Syria had crossed a “red line,” Obama went from saying he would “consult” Capitol Hill to actively courting its support.

The senior aides briefing reporters after Obama’s remarks suggested that he had largely settled on a course of action in an Aug. 24 National Security Council meeting, but did not make a final decision about using force until Friday.

No one — not Obama, not senior aides, not congressional leaders — had suggested securing congressional approval.

And then, sometime around 6 p.m. ET, Obama went for a 45-minute stroll around the South Lawn of the White House with Chief of Staff Denis McDonough, the aides said. During that walk, the president said that he wanted to go to Congress.

A two-hour meeting, from about 7 p to 9 p.m., followed with senior aides during which Obama to shared the same message. Some aides argued against that course-correction, the officials told reporters.

But by the time a National Security Council meeting wrapped up on Saturday, they were all on board, the aides said.

And they detailed the coming campaign to get Congress on board:

- Hammer home the potential threat to staunch ally Israel’s security

- Provide detailed intelligence about the alleged attack

- Underline that the United States ratified the Chemical Weapons Convention, and make a case that American legitimacy — not just his own — is at stake.

- Make the argument that failure to act could lead, one day, to terrorists acquiring chemical weapons from regimes like Assad’s — and turning them on America.