We should have withdrawn from Afghanistan even before bin Laden was killed - we should have withdrawn after we drove Al Queda out of Afghanistan (the fiasco at Tora Bora). The mission creep - from "getting bin Laden" to "nation building" was utter folly.
What we're seeing now in Afghanistan is... well it's what happens to a puppet - a marionette - when the puppet strings are cut. The puppet collapses.
The Afghan regime that we tried to create - our puppet, if we're honest about it - had no legitimacy, no real support. When we finally decided to cut the strings, the colonialist house of cards we erected, collapsed. Immediately.
Afghans will suffer unspeakable horrors as the Taliban reasserts control. Some of those horrors probably would not have happened, had the US not decided to play imperialist superpower, and impose a convenient regime on the Afghan people - but rather simply dealt with Al Queda, then left the Taliban alone. But once we decided to nation-build - to play God, bluntly - it was only a question of when this debacle would occur; when, not whether.
Our neocolonialism failed miserably in Southeast Asia. One would have hoped we had learned from that. Evidently not; maybe we'll do better in the future. And maybe (although I'm not optimistic) an Afghanistan without foreign imperialists (or "do-gooders," if you can't stomach "imperialists") will find its way to a more civil society - as Vietnam has done, no thanks to us.