Mr. Galbraith, the former UN envoy, sees the Afghanistan problem as a largely political affair, and rightly so, as counterinsurgency is largely a political issue. While military action is the vehicle through which political action is taken, the maxim from David Galula--that guerrilla war is eighty percent political and twenty percent military--still largely holds true (at least in a metaphorical sense). According to Galbraith, ISAF can clear Taliban forces from the cities, but an effective Afghan-led "hold" phase will likely fail, based on the government's past performance. (A recent article from the NY Times, reporting in Marjah, might lend credence to this claim)
Nevertheless, despite Nagl's more narrow focus on the operational aspects of counterinsurgency, I think he wins the debate with this quote:
The development of an Afghan government that is able to provide a modicum of security and governance for its people is necessary to ensure that the international community's security interests will be preserved without a continued major international troop presence. To achieve this objective, the coalition and its Afghan partners must build a state that reconciles a degree of centralised governance with the traditional tribal and religious power structures that hold sway outside Kabul.
This describes Afghanistan at its best (read: the mid-20th Century), where even then, it was rife with political corruption and coups, with failed experiments in liberal democracy. If we're expecting a nation-state that has some modicum of stability, success might be within our grasp. If we're expecting a Utopian society in Central Asia, we're guaranteed failure. (Which, fortunately, Nagl does not advocate)