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Despite roadblocks in forming an international coalition, Obama appeared undeterred and advisers said he would be willing to retaliate against Syria on his own.

“The president of the United States is elected with the duty to protect the national security interests in the United States of America,” White House spokesman Josh Earnest said.

Even before the vote in London, the U.S. was preparing to act without formal authorization from the United Nations, where Russia has blocked efforts to seek a resolution authorizing the use of force, or from Capitol Hill. But the U.S. had expected Britain, a major ally, to join in the effort.

Cameron told an emergency sitting of Parliament that the country should not be “paralysed” over its response to international crises in the wake of mistakes made in the run-up to the Iraq war.

He implored MPs to “force themselves” to watch harrowing videos of children suffering following a chemical weapons attack in Damascus last week which killed more than 300 ordinary Syrians.

However, Mr Cameron failed to win all-party support for a government motion supporting the principle of military action and late last night lost the vote by 272 to 285, a majority of 13 against.

A further vote of MPs will be necessary next week before Britain becomes directly involved in strikes.

The country’s response to the Syrian civil war is threatening to cause one of the most serious political rifts over the deployment of the Armed Forces since the Suez crisis in 1956.