Congress’s top leaders weren’t informed of the switch until just an hour or so before Mr. Obama’s Rose Garden announcement and weren’t asked whether lawmakers would support it. When the president’s chief of staff, Denis McDonough, announced the decision on a conference call with congressional committee leaders, some were so taken aback they seemed at first to misunderstand it.

Outside the U.S., Arab leaders privately urged the U.S. to bomb, but few backed Mr. Obama publicly. The United Kingdom pulled the plug on a joint operation two days after indicating to the White House it had the votes to proceed. Compounding the confusion, the same day a potential breakthrough emerged via a diplomatic opening provided by Russia, the administration sent a memo to lawmakers highlighting why Russia shouldn’t be trusted on Syria…

On Thursday, Aug. 29, the U.K. Parliament shot down Mr. Cameron, a major embarrassment to the British leader that raised pressure on the U.S. to seek other support. Opposition came from not only Labour but from Mr. Cameron’s own Conservative Party. Mr. Cameron threw in the towel, saying the British Parliament had spoken and the government would “act accordingly.”

The vote shocked Mr. Putin, who later told Russian state TV he thought legislatures in the West voted in lock-step, “just like the Communist Party of the Soviet Union.” Moscow’s alarm and frustration was growing as the move toward military action advanced, bypassing the U.N. Security Council where Moscow had veto power.