Ideally, the 66,000 American troops would already be leaving, and all of them would be out as soon as safely possible; by our estimate, that would be the end of this year. The war that started after Sept. 11, 2001, would be over and securing the country would be up to Afghanistan’s 350,000-member security force, including the army and police, which the United States has spent $39 billion to train and equip over a decade.

But there is a conflict between the ideal and the political reality. Mr. Obama has yet to decide how fast he will withdraw the remaining troops, and the longer he delays, the more he enables military commanders who inevitably want to keep the maximum number of troops in Afghanistan for the maximum amount of time.