As often is the case in Washington, the debate starts with the wrong facts. There are no "gains" worth mentioning in Afghanistan; except in a few districts, the COIN has been a failure. The overall picture is a stronger-armed opposition and no progress with Pakistan. Considering the coalition will have less resources next year, the only urgent problem is to enter a meaningful negotiation process with the Taliban and strike a deal to associate the Taliban to a new government.
Then, the real question is "how the withdrawal is going to impact the negotiations." If the pullout is too important, the risk is to see the Taliban deciding they just have to wait. If the withdrawal is only symbolic, the Taliban and, more importantly, the Pakistani army, which is supporting them, will think the U.S. army is still betting on a military solution. In this case, the military pressure on the coalition will quickly become unbearable. To escape this dilemma, one possible solution is to announce a relatively limited withdrawal (around 10,000 troops) and propose a cease-fire this autumn. This will show that the coalition is determined to leave and is serious about the negotiations, but not willing to exit quickly at any costs.