In autumn 2001, America's initial purpose in Afghanistan—which made perfect sense—was to destroy or incapacitate al Qaeda and punish the Taliban government that hosted it. This was accomplished 11 years ago. Today, the purpose of the U.S. mission is ill-defined, but clearly involves nation building. What the coalition desperately needs is an achievable, realistic endgame, not an indefinite timeline that commits thousands of U.S. troops to Afghanistan until or beyond 2024.
A common argument is that America and its allies must create an effective Afghan state that can rule the country and prevent the return of the Taliban and, by extension, al Qaeda. Aside from the fact that al Qaeda can exist anywhere, from Hamburg to Los Angeles, it's not at all clear that the coalition can either eradicate the Taliban or come close to creating an effective Afghan state.
As a Department of Defense Report declared earlier this year, "The Taliban-led insurgency remains adaptive and determined with a significant regenerative capacity, and retains the capability to emplace substantial numbers of [improvised explosive devices] and conduct isolated high-profile attacks that disproportionately field a sense of insecurity."