There is a Commons majority for air strikes against Isis in Syria - but Jeremy Corbyn will not be part of it. That was clear from David Cameron's statement on the need for military action and Corbyn's response. Cameron's most significant argument for intervention was that the threat to the UK from terrorism would only increase if it failed to act. The intelligence services, he said, had warned that Britain was already in the "top tier" of countries targeted by Isis. It was inaction, rather than action, that was the greatest risk.

Corbyn's response, consisting of seven questions, signalled that he does not share this view. Citing Afghanistan, Iraq and Libya, he quoted Barack Obama on the danger of "unintended consequences". His question on the risk of terrorist attacks in the UK and of civilian deaths in Syria showed that he believes both will be increased by UK air strikes. Yet a significant number of Labour MPs and shadow cabinet members share Cameron's view. Corbyn must now resolve by Monday whether to offer his party a free vote on the issue or whether to whip it against intervention (at the likely cost of frontbench resignations). The third option: Corbyn voting for air strikes seems unthinkable. It is a free vote that is the most likely outcome but in the view of one shadow cabinet member, failing to take a stance on a matter as grave as military action would make Labour "unfit for government".

Cameron, who was responding to the recent foreign affairs select commitee report opposing action, had made a multipronged case for intervention. He argued that the UK could make a unique military contribution through its Brimstone precision missiles (more accurate than those of any country), that there was a moral and strategic imperative for Britain to support its allies, the US and France; that a political process was underway (but action was needed before it concluded); that the threat from Isis would grow in the absence of intervention; that the UN resolution passed last week provided legal authorisation (along with the right to self-defence); that 70,000 Syrian opposition forces and Kurdish troops could fight Isis on the ground; that the government would contribute at least £1bn to post-conflict resolution; and that the west would not dismantle the Syrian state or its institutions (learning from the error of de-Ba'athification).

In response to Corbyn, Cameron later ruled out the use of UK ground troops. He maintained that "Assad must go" but argued for what he called an "Isil first" strategy. "We have to hit these terrorists in their heartlands now," he concluded.

The political test set by Cameron was to achieve a "clear majority" for military action. He warned that anything less would be a "publicity coup" for Isis. There are increasing signs that Cameron is close to meeting his aim. In response to his statement, Conservative MPs, including Crispin Blunt, the chair of the foreign affairs select committee (who previously opposed air strikes), Ken Clarke and Sarah Wollaston, announced that they would be voting for intervention (though the SNP and the Lib Dems will almost certainly vote against). But, as Cameron all but conceded, Corbyn will not be. The question facing the Labour leader is how he handles those in his party who intend to do so.