Speaking at the Council on Foreign Relations, Gen McChrystal, who commanded coalition forces in 2009-10 before being forced to resign after a magazine interview, said the US and Nato allies were “a little better than” half way to achieving their military goals.

The most difficult task would be to create a legitimate government that ordinary Afghans could believe in and that would balance the influence of the Taliban, he said.

“We didn’t know enough and we still don’t know enough,” he said. “Most of us – me included – had a very superficial understanding of the situation and history, and we had a frighteningly simplistic view of recent history, the last 50 years,” he said.

He added that while the choice to engage in Afghanistan may have been seen as the US acting on its right to defend itself after 9/11, the decision to invade Iraq two years later had both “changed the Muslim world’s view of America’s effort” and diverted some military resources that could have been put to good use in Afghanistan.