The decision by Mr. Obama appears to be a careful balancing of political interests and military requirements. The announcement enables the White House to say that slightly more than half of the American force — 34,000 troops — will be home by a year from now.

But the plan appears to give the military command in Afghanistan the flexibility it has long sought in determining the pace of the reductions. It appears that the command would be able to keep the bulk of the American force through the next fighting season, which ends in September and October.

American commanders want to keep sufficient forces — including troops, air power and medical evacuation units — to support the Afghan troops during this period. They also want to try to consolidate gains before rapidly drawing down American forces.