The boots-on-the-ground conundrum. The lesson from past interventions in Iraq and Libya, for example, is that jihadi terrorism thrives in a power vacuum, so there is little sense in striking in Syria to degrade Isis without confidence that some other force, more amenable to peace and civil reconstruction, can move in under the cover of airstrikes.

Cameron makes some brave assumptions here about the capabilities of the Syrian opposition and extrapolates very optimistically from limited progress made by Kurdish forces so far. This is the weakest link in his chain – an expression of wishful thinking and heroic ambition as much as a credible argument. It is hard to avoid the impression that he is deferring this aspect of the strategy, by far the trickiest one, until after the Tornados are in the air. But history suggests convenient deferral of this kind of plan is a fatal flaw – a shoot-from-the-hip response that undermines even the very best of intentions. A major cause for concern.