As President Obama meets with Afghan President Karzai this week, the future of American involvement is reaching a very important point.
Anyone who roots for an American defeat either isn't Americans or is a traitor, and defeatism is a form of rooting for defeat.
As to that American involvement in Afghanistan, that is because once certain parameters are set in that involvement, other actors influential in the various conflicts and/or rivalries in Afghanistan will start to commit to their policies, some of which might be less cooperative than otherwise should we not be looking ahead.
Pakistan's reaction is the single most important one, and if one understands their desire for influence and especially fear of encirclement, they don't have a right to destabilize that country either, if the TAP(I) pipeline is the best hope towards the end.
As to potential futures, we already know one because of what happened with the Soviet Army left in 1988; war and chaos.
Since that war and chaos led ultimately to Afghanistan becoming a safe haven for terrorism, that isn't in American interests, nor anyone else, save for some idiotic elements in Pakistan, and the worst elements in Iran.
China in the end, if they didn't mind watching us bleed a little, like Russia, mainly prefers that Muslim Central Asia be stable, since both have large Muslim populations that can be problematic as to being influenced.
India prefers an Afghanistan that is most definitely not a recruiting ground for insurgents to fight for Pakistan in Kashmir, the biggest single problem in winding the conflict down in good order.
Iran's main motive is to protect and also wield influence over ShiaHezzara in Afghanistan, and to protect itself against hostile incursions from there, American or not.
If there were a deal over Iranian nuclear weapons programs, therefore, Pakistan would be totally isolated diplomatically, in which there are carrots to offer them both, as to access to various pipelines of Central Asian natural gas, and trade routes along the "New Silk Road," all of which require a stable Afghanistan to work, and would in turn make Afghanistan "work" in the sense of revenue for the State to pay security forces to protect itself.
So beyond the level of troops in Afghanistan, in which one consideration for keeping a larger force is as a hedge against risk that can't be addressed once you draw down, something of a problem in Iraq now too, the diplomatic strategy of the United States of gettting Afghanistan's neighbors and near neighbors (The Stans will follow Ivan's lead in the end) is the most important feature of American policy going forward, especially using the Turkmenistan-Afghanistan-Pakistan(India) pipeline project known as TAP(I) to induce Pakistan to cooperat with India, and vice versa, if a deal with Iran over nuclear programs in return for no longer objecting to it either as a transit route would be the icing on the cake for making the most out of our troops sacrifices to acquire the strategic centering position that is Afghanistan.
Afghanistan is not Vietnam in that sense, as to having a much more central position between various interests, as to why to lose that analogy, since we wouldn't stop caring about that country, unlike what basically happened with Vietnam for twenty years, and then only in the context of other states viz China.