The countdown to Afghanistan withdrawal begins

The G8 is over; the NATO summit in Chicago is history. The world's leaders have moved on.

But spare a thought for David Cameron. Last night, as I passed through Chicago's O'Hare Airport, the Prime Minister was eating at a fast-food cafe, surrounded by fellow passengers, waiting for an American Airlines flight back to London. I was with Cameron's National Security Adviser Kim Darroch. "Austerity Britain," he smiled.

Apart from showing a common touch at the airport, what did the Prime Minister and the other NATO leaders achieve in Chicago?

Most importantly, the timeline for the withdrawal from Afghanistan was confirmed. One more fighting season to assault the Taliban. Then in the summer of 2013, NATO troops will shift to a purely supportive role. A year later, at the end of 2014, NATO forces fly home, their heads held high, the enemy in disarray.

That's the theory. And that was the problem with the NATO summit. It assumes that the Taliban won't fill the security vacuum left by the NATO withdrawal. But there are several military assessments suggesting that is exactly what will happen.

President Obama and the NATO leaders have one thing in common: they haven't the political will, the financial resources or the popular support to sustain the war for more than another year or two.

The French have already insisted their combat troops can't even wait that long. Forget NATO solidarity. Don't worry about the principle: "In together, out together."

The new French President is pulling his fighting forces out within the next few months.

So it's a matter of hoping that the Afghan security forces can step up as NATO steps down.

It's a remarkable fact that the world's greatest military alliance, after a decade of fighting, is left to hope for the best.