He said he was able to take the step now because of plans to reduce troop levels in Iraq more quickly than they are increased in Afghanistan and because of an enlargement that has increased the size of the active-duty Army to more than 547,000 troops, a level it had not been projected to reach until 2012.

The number of troops affected by stop-loss increased sharply under the troop increase for Iraq that President George W. Bush ordered in early 2007. Mr. Gates cautioned that “scores,” but not thousands, of soldiers might continue to be affected by the policy after March 2011 if they had skills that were particularly important to the war effort.

According to Pentagon statistics, 13,200 people are now under stop-loss orders: 4,458 in the Army National Guard, 1,452 in the Army Reserve and the rest from the active component. The Army said Wednesday that it would provide additional pay of $500 a month to those currently affected by the policy, retroactive to last October. Lt. Gen. Michael Rochelle, the Army deputy chief of staff for personnel, said that the policy had been “a vital tool” for successfully meeting the needs of fighting in Iraq and Afghanistan, but that Army leaders realized it “has caused hardship on soldiers and families.”

At its core, the stop-loss policy meant that all troops headed to Iraq and Afghanistan would remain in service throughout their unit’s deployment  even if the time on an individual soldier’s enlistment contract expired before the deployment ended. It also prevented the movement of soldiers who wished to remain in service but had intended to change positions within the military or attend a military school.

The Army has said the rule was required not just to sustain the numbers necessary to carry out two wars, but also to maintain continuity in leadership and cohesion within units that trained for and then were deploying to war.