The "mission" under Obama is to get as many Americans killed and wounded as possible, while wasting as much money as possible.
Yeah, that's it. *rolls eyes*
But yeah in theory the objective is to secure the current Afghan Government and prop it up as a stable American client state. But in practice I think it's to pull out as "honorably" as possible, and maybe (with the aid of Pakistan) broker a backroom peace deal between the Karzai government and the Taliban.
To be honest even though I opposed the Invasion of Iraq initially, looking back I would rather we just invaded Iraq and not Afghanistan.