The stunning parliamentary defeat on Thursday for Prime Minister David Cameron that 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.
Even on Thursday, a British summary of intelligence could say only that it was “highly likely” Mr. Assad’s forces were responsible for the use of chemical weapons. And many questions were raised, both Thursday night and in the days before, about whether the American assurances could be taken at face value, whether the expected riposte would accomplish any serious strategic or policy aim, and whether it might set off a worse regional conflict.
The government had seemed only days from joining the United States and France in cruise-missile strikes on Syrian targets, even though a United Nations Security Council resolution authorizing force was out of reach, because of Russia and China.
Mr. Cameron had yielded to the opposition Labour Party’s demands for a separate, second vote to authorize military force, to be held only after United Nations weapons inspectors finish their work in Syria. It was widely expected that Mr. Cameron would win Thursday night’s relatively meaningless vote on a motion supporting the notion that the chemical attack required an international humanitarian response that could involve military action. Instead, it was rejected, 285 to 272.
After the shocking defeat, Mr. Cameron was clear. “I strongly believe in the need for a tough response to the use of chemical weapons,” he said. “While the House has not passed a motion, it is clear to me that the British Parliament, reflecting the views of the British people, does not want to see British military action. I get that, and the government will act accordingly.”
The defeat, a sign of Mr. Cameron’s weakness, was also a tactical victory for the often-criticized Labour leader, Ed Miliband. But in larger terms, it is also a measure of Britain’s increasing isolation from its allies — both inside the European Union and now with Washington.
A strong anti-European wave on the British right led Mr. Cameron to promise a referendum on continued British membership in the European Union. And there is deep skepticism of Washington’s foreign policy, especially after the long, costly wars in Iraq and Afghanistan.
“The prime minister knew that the well had been poisoned by Iraq, but I don’t think he realized how much that was the case,” a Conservative legislator said, asking for anonymity. “They trust Cameron but not necessarily the advice he is being given.”
The vote took Britain into new constitutional territory, the lawmaker added, with Parliament effectively vetoing military action. Political recriminations are likely. But there was little disguising the humiliation for Mr. Cameron, who recalled Parliament specifically for a motion that he first watered down, then lost.
There is also a deep wariness here of using military force without the explicit backing of international law, expressed most clearly in a Security Council resolution, though without one, Britain participated fully in the NATO campaign to unseat Col. Muammar el-Qaddafi in Libya.
Mr. Miliband argued that, absent a resolution, the evidence should at least be put before the Security Council before any military action. The days of former Prime Minister Tony Blair, who cited “humanitarian intervention” as a casus belli, seem long gone in a country that now widely disparages him and his record.
Mr. Cameron’s troubles may not deter Mr. Obama from acting with the support of France, where legislative consultation is important but approval is unnecessary. But though President François Hollande appears ready, the French public, too, has doubts.
In the British Parliament on Thursday, the theme of doubt was foremost.
Paul Flynn, of Labour, said that prior uses of chemical weapons, as against the Kurds, had not drawn such a response. “Is not the real reason we are here today not the horror at these weapons — if that horror exists — but as a result of the American president having foolishly drawn a red line, so that he is now in the position of either having to attack or face humiliation?” he asked.
Sir Edward Leigh, a Conservative, said Britain should not allow American assurances to influence its decisions. He was particularly concerned with “the fate of the Christians” in Syria should Mr. Assad fall. And he asked whether the impact of military action would be sufficient to justify the likely deaths.
However, Malcolm Rifkind, a former foreign and defense secretary who is chairman of Parliament’s Intelligence and Security Committee, said, “There is no guarantee that a military strike against military targets will work, but there is every certainty that if we do not make that effort to punish and deter, these actions will indeed continue.”