David Milliband is the U.K. Foreign Secretary. That doesn't mean he is not an idiot. Check out his assessment of the state of his country's (and ours) engagement in Afghanistan.
The same could have been said (and actually was) said about a certain bunch of uncooperative colonials back in the 1775-1783 era.
Imperial occupiers don't win against determined local foes unless they (and their citizens) are willing to murder indiscriminately, pour vast armies into the project, and stay indefinitely. The U.S-U.K. alliance has shown some willingness to do the first, usually from the air. There is no sign that people in either country will indefinitely support a long war of attrition and occupation against people who fight because they have nothing to lose.
The Afghanistan project is FUBAR -- fowled up beyond all recognition. The question is not whether the U.S. will get out -- but when. It's the business of engaged people to hasten that day by educating ourselves about this war and creating political pressure for an end.