Theresa May’s decision to authorise British military action over the skies of Syria by royal prerogative rather than obtaining the backing of parliament was the wrong thing to do. Even if the prime minister thinks it was done for the right reasons. It was wrong because the government’s plans should have been articulated so that MPs could have had a chance to endorse – or reject – a motion to bomb Bashar al-Assad’s weapons factories. It was wrong because there was no emergency – an exception used when after a debate MPs retrospectively endorsed action against Muammar Gaddafi’s Libya. It was wrong because only prime ministers can recall parliament – and there was time to do so. It was wrong because decisions about how to police the unlawful use of weapons of mass destructive terror in Syria turn upon judgment rather than available facts.

Parliament is the best place to assess whether the use of military force serves the overall interests of a nation in such cases. This is especially true of a government without a majority of its own. Jeremy Corbyn’s resurrection of an old idea for a war powers act, which would force the PM not to authorise the active and large-scale deployment of British forces overseas without the approval of the House of Commons, ought not to be dismissed. But it should be accompanied by a wider recognition that the days of self-regulation of cabinet government are over. Observing the parliamentary convention would be better than creating an act where fractious disagreements over the precise nature of the circumstances in which the law is to be applied – especially in a situation as fluid and volatile as war – prevail.

This is a politically significant move by Mrs May. David Cameron became the first British prime minister to see his war plans foiled by parliament since 1782 when 30 of his own MPs in 2013 rebelled against the motion to bomb Syria. It was a humiliating and embarrassing moment but it reflected the weight of British public and political feeling against him. After the style of decision-taking on the road to the Iraq war of 2003, it was commendable that Mr Cameron sought to do something so unpopular as a matter of principle. A year later MPs backed his offensive against Islamic State in Iraq.

Mrs May took military action despite public sentiment. She has not carried voters or MPs. She ought to have persuaded the country, if she believed it, that Britain stood to gain in a punitive action of limited duration. If her defence was that this was unlikely to put Britain’s armed forces in harm’s way, she should have said so. Mrs May should have made parliament own the argument that western allies were making sure their red lines were addressed rather than trying to damage the Assad regime. In not doing so she makes her government culpable for the conduct of a murderous dictator. If Mr Assad decides to use nerve agents or chlorine again to kill and terrify his own people, the onus will be on Mrs May to react.

The Syrian president will see no threat to his hold on power and no change on the trajectory of the Syrian war. Even worse, ordinary Syrians will wonder why the west acted after 1,900 people were killed by chemical weapons when 400,000 have lost their lives to conventional weapons in the grisly civil war. Syria will continue as a zone of instability as long as Mr Assad remains in place, with minority Alawite rule – backed by Baathist-favouring Russia and Shia Iran – entrenched over Sunni Arab heartlands. This weekend’s action might prevent Mr Assad’s total victory, but it will not hasten his defeat. Mrs May should have been honest about this to both the public and parliament before dropping bombs.