AIR power can be enormously effective in damaging a group, but in the case of Isis that might not be enough. Modern airpower can obliterate almost any target it can see. As had recently been seen in the case of Jihadi John, if an Isis vehicle or military force is caught out in the open, air power can destroy it in seconds.

However, damaging Isis is very different from destroying Isis. The organisation’s great strength has been its ability to exert political control in a large area of eastern Syria and western Iraq. This control of a physical space lets it raise funds and recruit new members, including from Britain, far more easily than if it was always moving or on the run.

And that is where air power will face limitations. While it can damage Isis greatly in its area of political control, it cannot provide a replacement political authority. That will only happen if a force on the ground can physically occupy that space and asserts political control. That means a significant western force with outside Allies, perhaps including Iraqis and Saudis. But there seems little appetite for this. The Americans, French or British have little desire to send their own ground forces into another Middle Eastern country.

So as this seems an unlikely option, the ground troops with have to come from indigenous sources in Syria – a combination of the rebels fighting Assad, Kurds and other opposition forces. So far such forces have shown themselves only moderately capable. They have won some battles with Isis, but lost others. And they have already had air support in the past.

This is why so many people believe airpower alone will fall short. It has already been used in combination with forces fighting Isis, and it hasn’t triumphed. If it will be stepped up in use now, allied leaders will have to come up with a more intelligent and incisive plan of action. Saying what that plan of action is, will be very difficult.

Dr Phillips O’Brien is the director of the Scottish Centre for War Studies at Glasgow University

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