The events of the past week should leave no one in any doubt that the Iraq War is the defining event for the present political generation.

Neither 9/11 and the rise of Islamic extremism, nor even the economic crisis, which both have had more direct and visible consequences on the streets of Britain, have scratched scars as deep as the decision to go to war in 2003.

For MPs of the Adrian Mole generation, whose fuggy outlook was forged in the heady ideological confusion the followed the fall of the Berlin Wall, that vote in Parliament was the first time they had to take sides on an issue of global significance.

Politicians who voted to put the lives of British troops on the line still feel very raw, especially on the Labour side.

Although the principle of “humanitarian intervention” was never invoked as a pretext for war in Iraq, it soon became horribly plain that the conflict and its aftermath would make future intervention more difficult. And so it has proven with Syria.

David Cameron’s humiliation in the Commons last week will help build Ed Miliband’s reputation for ruthlessness. It builds our understanding of the pathology of the Labour leader, but it leaves the country without a foreign policy.

Those who oppose British military intervention in the Middle East must accept that there are consequences which flow from inaction as well as action. We will never know what would have happened if we had not gone to war in Iraq. Those of us who opposed intervention will never have to live with the consequences of keeping Saddam in place.

Ten years on, Westminster is a darker and more peculiar place: after all, Ed Miliband does not oppose the principle of intervention in Syria and yet voted against it.

And Britain is a darker place too. Over that decade, we have become a more sectarian nation. Divisions that began with the Iraq War have deepened. British Jews were expected to support the war because of the threat to Israel posed by Saddam, British Muslims to oppose it to because of the threat to their co-religionists.

The British left imploded in a festival of self-loathing. Loyalties were assumed and treacheries imagined.

The events in Parliament last week are part of the blowback from 2003. It is understandable that politicians are wary of making the same mistakes.

If Labour MPs are tempted to feel smug, they should ask themselves some simple questions. Is Assad committing atrocities against his own people? Do the Syrian people deserve our help? Do we have the capacity to help?

If the answer is yes to all three, then they need to ask one final question now that Parliament has voted against war: what do we do now?