For the British Government, it was the first defeat on military action since 1782. For Prime Minister David Cameron, it was a humiliation. The vote against bombing Syria in August 2013 continues to have consequences today. Why did it happen?
Problem one was timing. MPs were recalled to Westminster from the summer recess four days before they were due back anyway. The proximate cause: the use of chemicals in Syria, crossing President Obama's red line.
In those circumstances, Diane Abbott, the Labour MP, says, it was obvious why they were called back so suddenly: "If you voted on this that day, by the weekend British planes would be bombing Damascus.
"There was no ifs, no buts. There was only one reason to call us back early - to enable them to bomb at the weekend."
'No blank cheque'
David Cameron tweeted on Tuesday 27 August that MPs would return on Thursday, so government whips had just two days to ensure support. At the time of the Iraq vote in 2003, there had been weeks of preparation.
One government insider has told me that it was soon clear that Conservative backbenchers were unconvinced, and this message was relayed to Number 10.
Another, the then Foreign Office Minister Alistair Burt, says his canvass of about a dozen colleagues illustrated the extent of the problem: "I reported back I think you've got three who are very solid and are going to support this.
"I think you've got a couple who are persuadable and I think we have some people who are not going to vote for this."
This was problem two. The Conservatives are the largest party in the Commons, but depend on the Liberal Democrats to give the Government its majority. With Tory rebels, the chances of getting approval for military action depended on Labour.
Ed Miliband, grumble those inside Number 10, was problem number three. He and the prime minister met first on the Tuesday.
Government sources claim he offered "clarity", but the next day had begun "equivocating".
Douglas Alexander, the shadow foreign secretary, was also in those meetings. Asked if the official opposition was minded to support the government over Syria, he says Labour was "open" to it, denying that Mr Miliband misled Mr Cameron.
"It's not the job of the opposition to give a blank cheque to the government contemplating military action," he said.
"If you try and step back from the day-to-day discussions that were under way, you essentially had the attempt to shoehorn a timetable for legitimacy of the British Parliament into a timetable for the credibility of an American President."
The government changed the motion it was planning to debate, effectively ruling out British military action that weekend. That wasn't enough to persuade Labour. The opposition tabled its own amendment, requiring UN approval first.
'More political than military'
Were the differences so great? Problem number four may have been party politics. One government insider says there was "bugger all" between the two positions.
David Cameron's motion already promised a further debate before military action. Matters were no longer so urgent, so there was time to go to the UN.
Russia was likely to veto any move against Syria within the Security Council, at which point Britain would have had to decide whether to go ahead anyway. Ed Miliband's amendment did not rule out action in those circumstances. The real difference between the two sides may simply have been a matter of timing.
So why couldn't they hammer out a deal? Could it be that David Cameron didn't want to adopt Labour's amendment because it would look as if the opposition was determining Britain's foreign policy? Could it be, too, that Ed Miliband saw an opportunity to inflict a humiliating defeating on the prime minister?
Both leaders would dispute those interpretations of their behaviour during that one day in August. General - now Lord - Richards, was Chief of the Defence Staff until just a month before the debate. He's scathing about Labour's decision to oppose.
"What should have been a bi-partisan response - my goodness me, chemical weapons were being used - I didn't like the plan much, but you know that was what was on offer, you'd have thought that there would have been a consensus," he said.
"I can't remember the Labour Party saying the reason we're objecting is because this isn't a sound strategy. I can't even remember what their rationale was. So it seemed to me more political than military."
On Thursday 29 August, MPs voted 285-272 against action in Syria. It remains Britain's position to this day. Although Islamic State emerged from the chaos of civil war there, only in Iraq has the government asked for - and Parliament approved - military action.
The unanswered question is whether a vote one day in August 2013 can continue to determine British policy there.
Shaun Ley presents "The Syria Vote: One Day in August" on Monday 10 November at 8pm on BBC Radio 4. It will be available subsequently on BBC iPlayer.