It is more than two-hundred years since Parliament rejected a Prime Minister’s judgement on matters of war and peace.
But, despite last night’s Commons defeat, David Cameron may wake to a feeling of relief.
He had committed himself to military action against Syria, action opposed by around two-thirds of the population (if opinion polls are to be believed).
In rejecting his advice, Parliament may have done Mr Cameron a favour, keeping Britain out of a messy civil war -- but it may also have set a precedent that could bind his successors, and restrict their power over the armed forces.
Parliament’s right to be consulted on such matters was only conceded after the chaos surrounding the Iraq War in 2003.
Even so, it was seen as little more than a rubber stamp, backing a decision already taken in Downing Street.
And 48 hours ago, that’s exactly what we expected.
Until Ed Miliband changed Labour’s position, demanding UN inspectors report back before any binding vote on action against Syria.
Almost certainly, Russia would have blocked any resolution at the UN. And almost certainly, Labour would have (maybe reluctantly) backed British military action without a UN mandate -- there are few in Westminster comfortable looking the other way as Bashar al-Assad gasses his own people.
But, in moving against the Prime Minster’s stance, Mr Miliband emboldened critics of military action, including those on the Tory benches, tempting them into the No lobby.
If you want to see the continuing impact of Iraq on our politics, watch David Cameron’s response to the vote. He immediately acknowledged defeat, telling MPs “I get it”. Would Tony Blair have done the same thing? Or would he have told them “I hear you, but you’re wrong”.
Ed Miliband has been strengthened by this -- he took a significant gamble, a U-turn on his original position that earned a furious response from Downing Street, and could easily have been portrayed as weakness.
But if, as reported, he was applauded into the Labour whips’ office after the vote, his supporters might pause to remember the circumstances in which they’d been recalled to Parliament.
David Cameron, on paper at least, has been weakened. The Prime Minister advocated military action, and Parliament said no. Does Downing Street have the final say on British foreign policy anymore? Or have we now set a precedent that MPs, not ministers, make those decisions?
Embarrassing, certainly. But any military intervention in Syria will be complicated, messy. Mistakes will be made, innocent people will die. And it’s not clear if the kind of limited intervention being discussed in London and Washington will be enough to make a real difference in Damascus.
David Cameron may eventually see this as a lucky break.
For the people of Syria, though, luck remains in very short supply.