The joint US and European will to respond to last Wednesday’s chemical attack on the eastern outskirts of Damascus has received a blow after the British House of Commons voted narrowly 283 to 272 against a resolution that would have provisionally authorized British military intervention in Syria — a staggeringly rare defeat for a British government on a matter of foreign policy.

The vote comes as a blow not only to UK prime minister David Cameron, who suffered defections from nearly three dozen skeptical Conservatives as well as additional Liberal Democratic members of his own governing coalition, but also interventionists in the United States who are urging US president Barack Obama to launch an aggressive attack on the regime of Syrian president Bashar al-Assad.  It is very unlikely that the United States would proceed with unilateral military action without British support, which is unlikely to come anytime soon in light of Cameron pledge to respect the parliamentary decision:

I can give that assurance. Let me say, the House has not voted for either motion tonight. I strongly believe in the need for a tough response to the use of chemical weapons, but I also believe in respecting the will of this House of Commons. It is very clear tonight that, 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.

It’s a vote that has the potential to turn the US-UK relationship upside down, to turn Middle Eastern realpolitik upside down, to turn British politics upside down and even to turn US politics upside down.  For a sitting prime minister to lose a vote like this is a huge reversal in the relationship between an ever-more powerful British executive and an ever-more feeble parliament on issues like security policy and foreign affairs.

Most immediately, it means that a U.S.-led missile strike, which seemed imminent yesterday, will now be postponed until early next week, at the earliest, when chemical weapons inspectors from the United Nations have had an opportunity to provide their initial assessment of what happened in Ghouta and eastern Damascus.  The vote also comes after several news organizations reported that U.S. and allied intelligence agencies are assured that while the chemical attack came from pro-Assad forces, they are uncertain who ordered the attack amid indications that Assad and his top military brass were caught unaware.  Meanwhile, French president François Hollande has backed off earlier, more urgent calls for military action.

Cameron’s massive defeat does not necessarily preclude a vote next week after the United Nations reports back as to which party — and which chemical agent — is to blame for the horrific Damascus attack.  If the UN report, together with US and European intelligence, all points to Assad’s culpability, Cameron and Obama will have a much stronger case for an aggressive response, either inside or outside the United Nations Security Council.

Meanwhile, the vote is perhaps the largest political victory in Ed Miliband’s three-year tenure as leader of the Labour Party.  Miliband firmly opposed the resolution even after Cameron offered to submit to a second vote before authorizing military action, making today’s resolution essentially a vote for the principle of the British government’s potential military intervention.  The vote capped a tumultuous 24 hours in Westminster, with Cameron’s allies accusing Miliband of giving ‘succour’ to the Assad regime, which probably didn’t make it likelier that Labour would close ranks with the Tories over a potential Syria intervention.  It was a principled stand for Miliband and, though he’s closer to British public opinion on Syria than Cameron, it was also a courageous stand for a young opposition leader to oppose a sitting government on such a crucial matter of foreign policy.

Miliband’s line boils down to one sentence from his statement earlier today: ‘Evidence should precede decision not decision precede evidence’:

If the UN weapons inspectors conclude that chemical weapons have been used, in the eyes of this country and the world, that confers legitimacy on the finding beyond the view of any individual country or any intelligence agency.  What is more, it is possible that what the weapons inspectors discover, could give the world greater confidence in identifying the perpetrators of this horrific attack….

I am also clear that it is incumbent on us to try to build the widest level of support among the fifteen members in the Security Council, whatever the intentions of particular countries.  The level of international support is vital should we decide to take military action.  It is vital in the eyes of the world. That is why it can’t be seen as some sideshow or some moment, but actually an essential part of building the case if intervention should take place.

It wasn’t just Labour that opposed Cameron, though.  Top Tories remain skeptical, including many former ministers in the House of Lords, such as Douglas Hurd, who served as the UK foreign minister from 1989 to 1995 and Commons backbencher David Davis, a former shadow home secretary who was runner-up in the 2005 contest to lead the Conservative Party.

Philip Hammond, the UK defense secretary, blamed the government’s defeat on Iraq, which he claimed had ‘poisoned the well’ of public opinion, despite Cameron’s repeated assurances that the Syrian conflict is not the same as the war in Iraq.  Polls show the British public are incredibly wary of yet another intervention in the Middle East.  Justin Welby, the Archbishop of Canterbury, also came out against intervention in the House of Lords debate, and in the Vatican, Pope Francis also cautioned against Western intervention.

Aside from slowing down the U.S. rush to a military response, the consequences of the vote in the House of Commons will almost certainly embolden Congressional Republicans, including U.S. senators Ted Cruz of Texas and Rand Paul of Kentucky, to call for the U.S. legislative branch to have an equal say on U.S. intervention in Syria.