AP Photo

With the number of British and US casualties rising, the election results mired in complaints of fraud, and the Obama administration still reviewing its strategy, the war in Afghanistan is at once in crisis and in limbo. It is no wonder then that the Prime Minister should have started the new political term by trying to convince British opinion that this operation still has purpose and, more to the point, an end.

But the future of Afghanistan, of great geopolitical significance though that has to be, is by no means all that is at stake here. Looming behind the growing public debate about what this war is about and whether we British should be fighting it is quite another discussion: about the future or otherwise of Nato – the Western defence alliance that has...