Destroying Al Quaida and their host, the Taliban.
Al Quaida might be gone, but I believe Taliban are in power today and the US left in a not so glorious way after giving up fighting them.
Not just "giving up fighting them": when the US decided to leave, the taliban were in a stronger position than they were before the US invaded (eg they controlled a bigger part of the country and had much less opposition inside afghanistan). The war was already lost long before the US decided to leave.
