I have been in Kabul this past week. My new column of the FT has some observations about the challenges confronting US policy.

On every side, when one turns from goals to execution, intractable difficulties crowd in. Looming over everything is the question of US commitment. In choosing sides, much as they may dread the return of a fundamentalist tyranny, Afghans can be forgiven for asking who is more likely to be here in five years, the Americans or the Taliban. The prudent man hedges his bets.

Afghans know that the US policy debate revolves around exit. The Taliban explain it to those who are unaware. "They have the watches," they say, "and we have the time." Americans ask, when do we leave? But talk of an exit strategy for Afghanistan is 20 years premature. The country cannot be secured - and denied as a haven to enemies of the west - by force alone. Also required are far-reaching reform of its governance, perceptible improvements in living standards and wholesale reinvention of vital institutions such as the police.

This is not a one-year project, or a three-year project or even a 10-year project. Can the US credibly commit itself to underwrite this endeavour for as long as it takes? I doubt it. So do the Afghans. The irony, of course, is that the struggle might still drag on that long - at an even higher cost, ending ultimately in failure - because of that very lack of credibility.