LONDON — The stunning parliamentary defeat on Thursday for Prime Minister David Cameron, which led him to rule out British military participation in any strike on Syria, reflected British fears of rushing to act against Damascus without certain evidence.

By just 13 votes, British lawmakers rejected a motion urging an international response to a chemical weapons strike for which the United States has blamed the forces of the Syrian president, Bashar al-Assad.

The vote, and Mr. Cameron’s pledge to honor it, is a blow to President Obama. Like nearly all presidents since the Vietnam War, he has relied on Britain to be shoulder-to-shoulder with Washington in any serious military or security engagement.

But Mr. Obama’s efforts to marshal a unified international front for a short, punitive strike raised concerns about the evidence, reawakening British resentment over false assurances from the American and British governments that Saddam Hussein had weapons of mass destruction.