Veterans of intervention recognise the signs: a ghastly atrocity against civilians tips reluctant politicians and public opinion into action. The chemical attacks last week in the Damascus suburbs of Ghouta have echoes of the massacres in the Bosnian town of Srebrenica or more recently the imminent killing of civilians in Libya by the forces of the late dictator Muammar Qaddafi. In each case, caution is put aside as the moral and geopolitical costs of inaction appear greater than the costs of action.

Syria seems today to have reached that point. The regime’s belated agreement to let the UN weapons inspectors cross town to investigate what happened in Ghouta may reflect that it realises it risks isolating itself as a pariah. In the UK and France, commentators and politicians have made up their minds that it is time to do something about Syria. Government forces could only have instigated such an attack, with UN weapons inspectors just arrived in town, in order to seek a psychological knock-out blow against the rebels: if Bashar al-Assad can do this with impunity and still the West dithers, goes his implicit message to his opponents, then the rebels should give up on ever getting the western weapons and military support they had counted on.

He seems to have over-reached, although in the US there appears to remain greater uncertainty about the “what next”. This is not because Americans are less sure of the evidence but because of the burden of recent history and likely responsibility for the military action that follows. Iraq and Afghanistan have made both America’s military and its current government wary of interventions that have a horrible habit of expanding, offering no clear exit, and understandably get branded as “American-led”. Further, there is greater coverage in the US of the increasing Islamic radicalisation of the rebels.