In comparison to the debacle in Iraq, Afghanistan was something we were encouraged to believe we could take to the geopolitical bank, a successful and widely supported international intervention that perceptibly improved the lives of ordinary Afghans. That comfortable image has started to crumble.

First came the paroxysm of violence two weeks ago after a US army truck lost control and careered into a rush-hour traffic jam in Kabul. The spark that lit this fire was obvious but where did the tinder come from?