After a 45-minute walk Friday night, President Barack Obama made a fateful decision that none of his top national security advisers saw coming: To seek congressional authorization before taking military action in Syria.

The stunning about-face after a week of U.S. saber rattling risked not only igniting a protracted congressional fight, which could end with a vote against strikes, but a backlash from allies in the Middle East who had warned the White House that inaction would embolden not only Syrian President Bashar al-Assad but his closest allies, Iran and Hezbollah.

Aides said the decision was made by Mr. Obama and Mr. Obama alone. It shows the primacy the president places on protecting his hoped-for legacy as a commander in chief who did everything in his power to disentangle the U.S. from overseas wars. Until Friday night, Mr. Obama's national-security team didn't even have an option on the table to seek a congressional authorization.

The only real discussion was a plan to punish Mr. Assad for what the U.S. and others have called a chemical-weapons attack amid Syria's grinding civil war. The final question, policy makers thought, was how many targets to hit and when to tell the Navy destroyers in the eastern Mediterranean to open fire.

Yet Mr. Obama made no secret to aides he felt uncomfortable acting without U.N. Security Council backing. Current and former officials said his decision reflected his concerns about being seen as acting unilaterally—without political cover from Congress and without the U.K. at his side. Arab states, for their part, have offered little public support despite their private encouragement.