The prime minister said that while he still believed in a "tough response" to the alleged use of chemical weapons by Assad's regime, he would respect the will of Parliament.
The defeat was as dramatic as it was unexpected. At the start of the week, Cameron had seemed poised to join Washington in possible military action against Assad. The suspected chemical weapons attacks took place Aug. 21 in suburbs east and west of Damascus. The humanitarian group Doctors Without Borders has said the strikes killed 355 people.
Gruesome images of sickened men, women and children writhing on the floor drew outrage from across the world, and Cameron recalled Parliament from its summer break for an emergency vote, which was widely seen as a prelude to international action.
"The video footage illustrates some of the most sickening human suffering imaginable," Cameron told lawmakers before the vote, arguing that the most dangerous thing to do was to "stand back and do nothing."
But the push for strikes against the Syrian regime began to lose momentum as questions were raised about the intelligence underpinning the move. During a debate with lawmakers, he conceded that there was still a sliver of uncertainty about whether Assad truly was behind the attacks.
"In the end there is no 100 percent certainty about who is responsible," Cameron said, although he insisted that officials were still as "as certain as possible" that Assad's forces were responsible.
That was not enough for Britain's Labour Party, which is still smarting from its ill-fated decision to champion the invasion of Iraq in 2003. The party announced its opposition to the move despite Cameron's concessions, which included a promise to give U.N. inspectors time to report back to the Security Council and to do his outmost to secure a resolution there.
He also promised to give lawmakers a second vote in a bid to assuage fears that Britain was being rushed into an attack.
Cameron's impassioned pleas and hours of debate failed to dispel lingering suspicions that what was billed as a limited campaign would turn into an Iraq-style quagmire, and the prime minister lost the late-night vote 285-272. Some lawmakers shouted: "Resign!"
Tony Travers, the director of the government department at the London School of Economics, said Cameron had clearly miscalculated when he brought Parliament back early from its summer recess. He said the move had been unpopular even within Cameron's Conservative Party.
"Clearly this will be seen as a defeat, it suggests he got the politics wrong, both with the opposition and with some members of his own party," Travers said. "It's not great, it's not brilliant, nor is it the end of the world for him. He's lost votes before. It doesn't necessarily stop them taking further action, but they are going to have to start again really."
He said there was "not a lot" of public support for British military activity in Syria.
Defense Secretary Philip Hammond confirmed that British forces would not be involved in any potential strike, something he said would doubtless upset Washington — and please Assad.
"It is certainly going to place some strain on the special relationship," Hammond told BBC radio. "The Americans do understand the parliamentary process that we have to go through.... Common sense must tell us that the Assad regime is going to be a little bit less uncomfortable tonight as a result of this decision in Parliament."
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