A defeat in Parliament on matters of peace and war is without modern precedent.
The question is, what does it mean?
First and foremost - Britain will not take part in any military attack on Syria.
America may press ahead regardless but the principal advocate of a tough response to the Assad regime has been neutered.
The prime minister has lost control of his own foreign and defence policy and as a result he will cut a diminished figure on the international stage.
Some strong advocates of the transatlantic relationship worry that America may now question the value and reliability of Britain as an ally.
It is - perhaps - here at home, though, that David Cameron will feel the most political pain.
The rupture with his own party which he did so much to try to repair is back on public display.
What's more, the man who is determined to replace him - Ed Miliband - has been given the opportunity to disprove the claims that he is weak - and will walk taller as a result.
The repercussions of this vote could be felt for a very long time to come.