What Andrew Exum sees as the most likely endgame in Afghanistan (he also provides worst-case and best-case scenarios):
The most likely scenario in Afghanistan..is one in which the United States and its allies gradually tire of a costly counterinsurgency campaign and transition to a more limited engagement that, while not meeting many of the strategic goals articulated by the president in March, allows the United States and its allies to still influence affairs in Central Asia and prevent a total return of the Taliban and its allies to power in Afghanistan.
In this scenario, most U.S. allies withdraw their forces from Afghanistan in the next 18 months as the war becomes more exclusively a concern of the United States and its Afghan partners. Spurred by popular displeasure with the war in his own party, the president directs the commanders in Afghanistan to reduce the presence of U.S. general purpose forces and to shift the mission away from a large-scale counterinsurgency campaign to foreign internal defense (FID) making better use of U.S. Special Forces and other special operations forces. A limited and short-term “surge” into Afghanistan precedes this transition, with the goal of rapidly training more ANSF. A combination of U.S. and allied airpower and direct-action special operations support this limited surge.