The title of this post is wrong. Afghanistan has never been and will never be a nation except in the short-lived case of control by a ruthless local chief who happens to have, for an instant in time, overwhelming force backed by financial deals and power sharing. He too shall pass.
It is still a strange turn of events that led the US from its principal, strategic reason for striking in the first place - severely damaging Al Qaida and denying it and other jihadist groups from establishing a base in the future. In that, the US policy has succeeded. Nation building has nothing to do with it and still does not - or at least should not.
Once US military forces are withdrawn, the infighting will begin again with local forces manipulated by Russia, Pakistan, India and Iran. In January I suggested that the best policy for the US was withdrawal of most combat troops and a reversion to tactics that reflected the only legitimate and achievable US strategic objective - preventing the use of Afghanistan as a base for terror operations and so degrading and destroying the central command of Al Qaida that it would be unable to mount any trans-national operations. The latter has been achieved and can be maintained through the use of much smaller, quickly deployed ground units and air strikes. Instead, a surge was initiated to defeat the Taliban and create a nation around President Karzai. This is simply not going to succeed.
The US will need to find a political solution and leave with grace - unlike the English and Russians before them. Afghanistan is simply not worth the candle.