US President Barack Obama's aides were stunned at what their boss had to say when he summoned them to the Oval Office at 7pm Friday, on the eve of what they believed could be a weekend when American missiles streaked again across the Middle East.
In a two-hour meeting of passionate, sharp debate in the Oval Office, he told them that after a frantic week in which he seemed to be rushing toward a military attack on Syria, he wanted to pull back and seek congressional approval first.
He had several reasons, he told them, including a sense of isolation after the terrible setback in the British Parliament. But the most compelling one may have been that acting alone would undercut him if in the next three years he needed congressional authority for his next military confrontation in the Middle East, perhaps with Iran.
If he made the decision to strike Syria without Congress now, he said, would he get Congress when he really needed it?
"He can't make these decisions divorced from the American public and from Congress," said a senior aide, who spoke on the condition of anonymity. "Who knows what we're going to face in the next 3½ years in the Middle East?"
The Oval Office meeting ended one of the strangest weeks of the Obama White House, in which a president who had drawn a "red line" against the use of chemical weapons, and watched Syrian military forces breach it with horrific consequences, found himself compelled to act by his own statements.
But Obama, who has been reluctant for the past two years to get entangled in Syria, had qualms from the start.
Even as he steeled himself for an attack this past week, two advisers said, he nurtured doubts about the political and legal justification for action, given that the UN Security Council had refused to bless a military strike that he had not put before Congress. A drumbeat of lawmakers demanding a vote added to the sense that he could be out on a limb.
On Sunday, the White House issued a statement dismissing the need to wait for UN investigators because their evidence, the statement said, had been corrupted by the relentless shelling of the sites.
By Monday, Secretary of State John Kerry, who had long advocated a more aggressive policy on Syria, delivered a thunderous speech that President Bashar al-Assad was guilty of a "moral obscenity."
By midweek, administration officials were telling reporters that the administration would not be deterred by the lack of an imprimatur from the Security Council, where Syria's biggest backer, Russia, held a veto.
Beyond the questions of political legitimacy, aides said, Obama told them Friday that he was troubled that authorising another military action over the heads of Congress would contradict the spirit of his speech last spring, in which he attempted to chart a shift in the US from the perennial war footing of the post-September 11 era.
All of these issues were on Obama's mind when he invited his chief of staff, Denis R McDonough, for an early evening stroll on the south lawn of the White House. In the West Wing, an aide said, staff members hoped to get home early, recognising they would spend the weekend in the office.
Forty-five minutes later, shortly before 7pm, Obama summoned his senior staff to the Oval Office.
"I have a pretty big idea I want to test with you guys," he said to the group, which included McDonough and his deputy, Rob Nabors; the national security adviser, Susan E Rice, and her two deputies, Antony Blinken and Benjamin J Rhodes; his senior adviser, Dan Pfeiffer, and several legal experts to discuss the War Powers Resolution.
The resistance from the group was immediate. The political team worried that Obama could lose the vote, as Cameron did, and that it could complicate the White House's other legislative priorities. The national security team argued that international support for an operation was unlikely to improve.
At 9pm, the president drew the debate to a close and telephoned Kerry and Secretary of Defence Chuck Hagel to tell them of his plans.
1941 The last time that a formal declaration was adopted by Congress was in 1941, when the US entered the second world war.
1973 The War Powers Resolution, initiated in response to the deeply unpopular Vietnam War, in theory requires the president to seek authorisation from lawmakers for any military intervention lasting beyond 60 days.
1992 Operations in Somalia and Haiti (1994) also took place without congressional approval.
1995 President Clinton ordered 20,000 troops into Bosnia-Herzegovina despite Congress's failure to agree on several draft resolutions.
1998 Clinton approved cruise missile strikes on Afghanistan and Sudan following the twin bombings of US embassies in Kenya and Tanzania.
1999 Clinton ordered air strikes without congressional approval, hitting Yugoslavia for 78 days in the Kosovo conflict.
2003 President Bush sought and received authorisation under the resolution before launching the invasion of Iraq.
2011 Obama justified military intervention in Libya on the grounds of a UN Security Council resolution despite Congress' request that it be consulted.