It might at least be hoped that the lessons of this debacle will be learned. The main one is so obvious: an over-militarised foreign policy that embraces unrealistic objectives is liable to fail. Yet, as Mr Kagan and many others have shown, this appears to be more a feature of American foreign policy than a bug.
For some, the answer is simple. The Taliban are so odious, so violent, repressive and untrustworthy, that America should have as little as possible to do with them. Handing out aid or initiating formal diplomatic ties would only strengthen the new regime, the argument runs; better to leave it to stew. Others make the opposite case. It is no use lamenting the outcome of the war, they say: if America wants to retain any influence in Afghanistan, it had better start engaging now, before China, Iran and Russia turn the country into their client.
The Taliban, of course, may prefer pious penury to growth and development. But to the extent that Western help is wanted, it should be dished out in small doses, tied to specific concessions.