It is just 13 months since David Cameron became the first prime minister to lose a vote on a matter of peace and war since 1782. After his defeat over potential military action in Syria, many spoke as if an epochal shift comparable to Suez had occurred. William Hague, the then foreign secretary, considered resigning and told colleagues that he didn’t want to represent “a country that is not prepared to act”. Paddy Ashdown lamented that the UK had lurched “towards isolationism”. One Conservative MP told me after the vote: “We won’t be involved in military action for the foreseeable future and certainly not in this parliament.”

But a year later, Westminster is again contemplating the use of force, this time to halt the murderous advance of Islamic State. The assertion of the Foreign Secretary, Philip Hammond, that the UK would play a “leading role” in the coalition assembled by the US was the clearest signal yet that parliament will soon be asked to give its approval to air strikes against the group in Iraq. One Downing Street strategist pointedly notes that Tornado fighter jets are already performing reconnaissance missions in the country – and they could easily shift to an offensive role.

While acknowledging the legal, technical and political obstacles, No 10 isn’t even dismissing possible action in Syria. Conservative sources downplay suggestions that parliament could be recalled as early as 25 September (“We’re not at that stage yet”) but some Tory MPs can already see the political advantages. “It would blow the Ukip conference out of the water,” notes one with Machiavellian relish.

After Barack Obama’s skilful forging of an international coalition, one that crucially includes ten Arab states, Cameron’s task is to construct a domestic equivalent. Having broken one of the first rules of parliamentary politics last year – that prime ministers shouldn’t call votes they are going to lose – the Conservatives are acting with greater diligence this time. Party whips, now led by Michael Gove, have canvassed opinion in the tea rooms and have concluded that Cameron’s own side, even when combined with the Liberal Democrats, will not be big enough to guarantee him victory.

Some of the 30 Tory MPs who voted against intervention in Syria are prepared publicly to support military action against Islamic State. One of the rebels, Sarah Wollaston, tells me: “Last time round, we didn’t have Arab League backing and we do this time. The intervention has to be led by local and regional powers; otherwise, it’s just seen as a vendetta against Muslims.” But most Tory rebels remain unmoved. John Redwood, who abstained on the government motion last year, bluntly says: “I am not persuaded that we should be bombing Iraq or Syria.”

It is Labour that will determine whether Cameron becomes the fourth successive prime minister to preside over military action in Mesopotamia. After thwarting the PM last summer, in what the Tories denounced as an act of betrayal (Labour sources maintain that no “blank cheque” was ever signed), the opposition is regarded with suspicion and enmity. But there is as yet no evidence that the two parties’ positions will diverge. Beyond opposing the deployment of “boots on the ground” (as the government does), Labour strategists emphasise they are “ruling nothing out”. One tells me the party has adopted a “bipartisan approach”, resisting the temptation to seek political advantage from the government’s often conflicting messages. Ed Miliband has supported the arming of the Kurds, British logistical support and, most significantly, targeted air strikes against Islamic State by the US.

Less than eight months before a general election, when partisan tensions are normally at their highest, some in Labour are even moved to rare praise for Cameron. One shadow cabinet minister commends his “patient multilateralism”, and another senior figure notes: “This is not a macho moment like in August 2013. He’s not doing grandstanding.”

Some of the loudest interventionist voices come from the opposition benches. Peter Hain, the former Labour cabinet minister, tells me that the government should consider “opening back-channels” to the Assad regime in order to enable air strikes in Syria. Ann Clwyd, who served as Tony Blair’s special envoy to Iraq, says that it is “irresponsible” to rule out the use of ground troops ultimately to repel Islamic State. Were Miliband to oppose air strikes, he would face a rebellion greater than that over Syria, when four Labour MPs (Clwyd, Ben Bradshaw, Meg Munn and John Woodcock) refused to vote against the government motion.

His likely support for them should not come as a surprise. Not since 1935, when Ernest Bevin denounced his leader George Lansbury for “hawking your conscience round from body to body asking to be told what to do with it”, has Labour been a pacifist party. Unlike the extra-parliamentary left, Michael Foot was resolute in his support for the Falklands war in 1982. And though labelled as a peacenik due to his opposition to the 2003 Iraq invasion and his sceptical stance on Syria, Miliband, too, stands in this internationalist tradition. In March 2011, invoking Britain’s failure to act during the Spanish civil war, he supported intervention in Libya to prevent a massacre of the innocents in Benghazi.

Rather than the dawn of a new age of isolationism, the Syria vote is now identifiable as an accidental pause. After Cameron’s defeat, senior Labour figures confessed that they were surprised by his abrupt rejection of military action. Those events raised the bar for parliamentary approval of intervention. But it is a bar that the Prime Minister, humbled by experience and shorn of bluster, is now in a position to clear.