Who will take on the un-Islamic State?

A year ago this week something unexpected happened. The Government was defeated, by 13 votes, on a motion to consider military action in Syria after a further vote. David Cameron had recalled Parliament so that it could approve British forces taking part in US-led punitive strikes against Bashar al-Assad's regime for its use of chemical weapons. Ed Miliband, reading public opinion rather better than the Prime Minister, started to lay down conditions, including, late in the day, the requirement of a second vote before the planes actually took off.

This rather negated summoning MPs from door-knocking duties in their constituencies, but Cameron had enough trouble with the isolationist tendency of his own MPs and decided that cowardice was the better part of leadership. Then Miliband decided to oppose the Government motion, although it conceded everything he wanted and the previous day he had supported military action in principle. Ten years after the Iraq war, its trauma was still raw: Miliband hoped to impress voters wary of further entanglements in the Middle East, but he did not expect to win the vote.

When the tellers read out the result, therefore, he was obliged to cover his shock by declaring victory. The vote had been about "preventing a rush to war", he said, although the "rush" had already been taken out, and the effect of the vote was to rule out military action altogether. Cameron said the will of the House was clear – "I get that" – and that he would not make another proposal for action in Syria.

Outright opposition to military force would have been an honourable position. Miliband could have argued either that Assad's use of chemical weapons ought to be punished and deterred, or that Western air strikes would only make matters worse. What was deplorable was to argue both positions, one after the other.

Then chaos theory kicked in. The flap of a butterfly's wings in the Labour leader's office triggered a chain of events around the world. Responding to the Commons vote, Barack Obama announced that the US Congress would be asked to approve strikes against Assad. When it became clear, a few days later, that the President didn't have the votes, Obama had to abandon the idea. Most unexpected of all, the Russians responded to this show of Western weakness in the way deterrence theory said they shouldn't, and persuaded Assad to hand his chemical weapons to the UN.

Was that a triumph for Miliband or a disaster? He had, unexpectedly and accidentally, influenced world events, but for good or ill? The people to be wary of in this debate are those who are sure of the answer, and for whom every twist and turn of the conflict confirms what they thought already.

For the rest of us, the debate over intervention has swirled and reversed repeatedly. It was Paddy Ashdown, leader of the party that later stridently opposed intervention in Iraq, who gave voice to the lesson from genocide in Rwanda in 1994. He helped shame the Tory government for its failure to act over Bosnia, and to prod the Labour opposition to adopt an "ethical" position. That led to Kosovo, in which Tony Blair chivvied a reluctant US president, supported by the liberal left, but after 9/11 atavistic anti-Americanism kicked in, and we all know what happened to Labour after Iraq.

Now everything has turned around again. Obama was elected to "end the war" (Mission Not Yet Accomplished), while France, the cheese-eating surrender monkeys of Iraq 2003, turned into the cheese-eating attack monkeys of Libya 2011. Last week François Hollande, who had briefly been Miliband's economic template, turned out not to be his foreign-policy model either. "If, one year ago, the major powers had reacted to the use of chemical weapons, we wouldn't have had this terrible choice between a dictator and a terrorist group," he said. I don't follow that argument, but with Hillary Clinton saying something similar in The Atlantic, Miliband's diplomatic nightmare is taking shape.

No wonder the Labour leader has stayed holed up in the south of France, doubtless working on his conference speech. Like Gordon Brown as Chancellor, who worked for one big event a year, the Budget, Miliband devotes himself to the set piece. "Yes, he doesn't have a country to run and long may that be the case," said one grumpy Downing Street official.

But Miliband's absence has allowed Cameron too much space to swither. As our ComRes poll today confirms, public opinion, including liberal left opinion, is swinging back to a willingness to commit British forces – in the air at least – to stopping Unislamic State, as we should call it.

We could do with some leadership, but both Cameron and Miliband are too cowed by the Syria vote debacle a year ago, itself a product of regrets about Iraq a decade earlier.