A Commons vote on Syrian airstrikes puts both May and Corbyn in a tricky situation

“I get that, and the government will act accordingly,” a crestfallen David Cameron told the Commons in August 2013 after MPs voted against military strikes against the Assad regime for using chemical weapons against its own people.

But Cameron did not need to abide by the MPs’ decision. Nor did he have to consult Parliament in the first place. The power to go to war is invested in prime ministers, acting on behalf of the monarch under the Royal Prerogative.

Cameron believed that Tony Blair had set a precedent by winning the approval of the Commons for the disastrous invasion of Iraq in 2003. The 2013 vote was a landmark. It was the first time MPs had blocked military action. It also entrenched Blair’s precedent.

As Donald Trump warns Russia to “get ready” for attacks in Syria, Theresa May is now weighing the arguments for joining the US and France in air strikes against Assad after Saturday’s horrific chemical weapons attack in Douma in Eastern Ghouta. Several Conservative and Labour MPs are urging her to go ahead without parliamentary approval. MPs are in a two-week Easter recess and do not return until next Monday, by when Trump and Emmanuel Macron may have pressed ahead with action. May could recall Parliament for an emergency session but there is no sign of this happening. Indeed, the recess may suit May.

Her instinct will be to “be there” with the US and France. She has just pleaded for and received strong support from Britain’s allies over the nerve agent attack in Salisbury. She cannot divorce from her judgment a desire to show the UK is still a player on the global stage as Brexit looms.

Yet May is naturally cautious. She wants to uphold the ban on the use of chemical weapons, but rightly wants to act in accordance with international law. Hence her emphasis on the need for “evidence”. She also wants to avoid a humiliating Commons defeat like Cameron’s in 2013, which would set back her recent political recovery.

However, winning a Commons vote might be easier than she thinks. Labour MPs tell me they estimate that least 30 of them would support action now, saying the number could easily rise to 50. Some who opposed action in 2013, when Ed Miliband infuriated Cameron by deciding not to back air strikes, either regret their vote then or have changed their minds given what has happened in Syria since. Although 30 Tory MPs voted against action five years ago, Labour supporters of air strikes are convinced they would give May a majority now.

Winning a mandate would strengthen her position and the case for intervention. Despite the long shadow still cast by the war in Iraq, May does have a case: if the West turns a blind eye, Assad could continue to use chemical weapons with impunity. Moreover, the success in limiting their use since the First World War will be lost; could they become the norm, with a chilling new arms race between barbaric leaders prepared to deploy them? At some point, a red line will have to be drawn and stuck to. But the case should be tested in Parliament before it is too late.

Corbyn will oppose action. He has an honest and consistent approach to such matters. But Syria is a tricky issue for him as well as May. Corbyn calls for a political solution in Syria, but as the seven-year civil war draws towards a close in Assad’s favour, there is little or no prospect of one. With 30-50 Labour MPs feeling strongly enough to defy a three-line whip, Corbyn may decide not to impose one.

There is a new mood inside the Parliamentary Labour Party. After keeping their heads down since his remarkable performance at last year’s election, Corbyn’s MP critics have started to challenge him over his refusal to blame Russia unequivocally for the Salisbury attack and his uncertain response to allegations of antisemitism inside his party. Now Syria adds another dimension, and a common thread with Salisbury – Corbyn’s stance on Russia.

Events in Syria highlight the need to clear up the confusion over Parliament’s role in military action. In 2011, the Coalition government promised a new law to codify the emerging convention that MPs should approve it. But nothing happened and the idea was finally dropped in 2016, on the grounds that it would limit the government’s freedom to act.

There is a way to square the circle: a law to replace the Royal Prerogative with a parliamentary one, that would set out certain conditions where ministers could act swiftly without a Commons vote. Of course, the government would need the flexibility to react quickly to an immediate threat to the UK or its citizens and in a humanitarian crisis.

But MPs should normally be able to ask questions before the event, rather than rubber-stamp action retrospectively as May appears to want to happen now. Questions such as “what is the plan for tomorrow?”, the biggest lesson of Iraq. Today, one would be: what happens if the unpredictable, unreliable Trump calls time after one wave of air strikes with the UK and France, and Assad then launches another chemical attack? Such matters need to be addressed, and Parliament is the right place to do it.