Afghanistan was vengeance for the 9/11 Twin Towers killings.  

You do remember Osama bin Laden and his training camps bought from the Taliban, don't you?

Then we decided that a nation which has been a corrupt bunch of tribes for centuries could become 'democratic' if we just killed enough of the 'bad' people while providing millions of dollars of bribes to the 'good' people.

Time to cut our losses?

Or do a Nixon.  Declare 'victory' and leave.  Worked in Viet Nam.