...The U.S. does not have United Nations support to strike Syria, and U.N. Secretary-General Ban Ki-moon has urged restraint. ‘‘Diplomacy should be given a chance and peace given a chance,’’ he said Thursday.

Expected support from Britain, a key ally, evaporated as Parliament rejected a vote Thursday endorsing military action in Syria. And diplomats with the 22-nation Arab League said the organization does not support military action without U.N. consent, an action that Russia would almost certainly block. The diplomats spoke anonymously because of rules preventing them from being identified.

‘‘Presidents always need to be prepared to go at it alone,’’ said Rudy deLeon, who was a senior Defense Department official in the Clinton administration.

‘‘The uninhibited use of the chemical weapons is out there, and that’s a real problem,’’ said deLeon, now senior vice president of security and international policy at the liberal-leaning Center for American Progress in Washington. ‘‘It can’t be ignored, and it certainly creates a dilemma. I think (Obama) had to make the red-line comment, and so Syria has acted in a very irresponsible way.’’

The nearly nine-year war in Iraq that began in 2003, which Obama termed ‘‘dumb’’ because it was based on false intelligence, has encouraged global skittishness about Western military intervention in the Mideast. ‘‘There’s no doubt that the intelligence on Iraq is still on everybody’s mind,’’ deLeon said.

Both Republican George H.W. Bush and Democrat Bill Clinton had U.N. approval for nearly all of their attacks on Iraq years earlier. Even in the 2003 invasion, which was ordered by Republican George W. Bush, 48 nations supported the military campaign as a so-called coalition of the willing. Four nations — the U.S., Britain, Australia and Poland — participated in the invasion.

The U.S. has relied on NATO at least three times to give it broad foreign support for military missions: in bombarding Bosnia in 1994 and 1995, attacking Kosovo with airstrikes in 1999 and invading Afghanistan in 2001.

Only a few times has the U.S. acted unilaterally — and only then to respond to attacks or direct threats against Americans...

MPs have rejected possible UK military action against Syrian President Bashar al-Assad's government to deter the use of chemical weapons.

David Cameron said he would respect the defeat of a government motion by 285-272, ruling out joining US-led strikes.

The US said it would "continue to consult" with the UK, "one of our closest allies and friends".

France said the UK's vote does not change its resolve on the need to act in Syria.

Russia - which has close ties with the Assad government - welcomed Britain's rejection of a military strike.

The prime minister's call for a military response in Syria followed a suspected chemical weapons attack on the outskirts of the capital Damascus on 21 August, in which hundreds of people are reported to have died.

The US and UK say the Assad government was behind the attack - a claim denied by Damascus, which blames the rebels.

Assad said Syria would defend itself against any aggression.