On Thursday, I painted both optimistic and pessimistic pictures of the consequences that could flow from the bombing of ISIS.  The key to making the former (which is the less likely outcome) real is to recognise that behind the problem of ISIS lies, as Rory Stewart put it in the Commons yesterday, “not a military problem but a diplomatic and political problem”.  Defeating ISIS requires the support of the Sunni Muslims, just as the defeat of Al Qaeda in Iraq did ten years or so ago.  That means arming the Kurds, improving Iraq’s army, curbing corruption in its government – and letting America and Britain’s Sunni Arab allies take the military lead.  David Cameron recognised as much in his speech: “To be absolutely direct, I am not claiming that by air strikes alone we can roll back this problem,” he said.

However, many Sunni Muslims in Iraq and Syria will put up with ISIS, for all its barbarity, if the alternative is the Iran-backed Assad regime in Syria and the Shia part of the Iraqi army, which is substantial.  Were Saudi Arabians and Quataris and other Sunni Arabs to stop funding ISIS, and were Iran and Russia to manage the removal of Assad, it is possible to imagine the emergence of a government in Damascus acceptable to mainstream Sunnis, alongside that of a similar government in Baghdad.  Sunni support for ISIS would then dwindle away, just as it did for Al Qaeda roughly a decade ago.  Admittedly, this glittering prospect is unlikely to be realised, despite the startling improvement of America’s relations with Iran.  But one thing is certain.  If it, or something like it, does not happen, bombing ISIS will achieve little good, at best.

And since Syria is part of the problem, it must also be part of the solution.  Arming the Free Syrian Army (FSA) has always been desirable in principle but questionable in practice – given the way in which weapons given to moderate Syrian groups often end up with extreme ones.  But if the Kurds are to be supplied and the Iraqi army supported (again), there is clearly a case for reviewing the issue.  (There are claims that the FSA has been more effective with the weapons and training it has received than is usually believed.)  At any rate, a Sunni-led war against ISIS – indeed, any war against it – cannot stop at a border that no longer exists: Sykes-Picot is a dead parrot.  It will be claimed that such action could only help Assad.  But although Assad and ISIS are usually presented as enemies, they are, in some ways, allies.

This is because both have a mutual interest in crushing moderate Sunnis – ISIS, in order to convince the mass of the Sunni population that they provide the only protection from Iran-backed Shi’ites and Alawites; Assad, in order persuade to the western democracies that he offers the only practicable help to hand against ISIS, and is thus the lesser of two evils. Indeed, the selling of oil to Assad by ISIS has been complemented by military co-operation by both against moderates.  Yesterday’s Commons motion deliberately excluded Syria, no doubt to keep Labour onside.  But the issue of military action against ISIS within it as well as outside it can’t be ducked forever.  If this were to work it would have to be Arab and Sunni Muslim-led, for the reasons given.  At the moment, this is a distant prospect.