The number of soldiers forced to remain in the Army involuntarily under the military's controversial "stop-loss" program has risen sharply since the Pentagon extended combat tours last year. Defense Secretary Robert M. Gates was briefed about the program by Army officials who said that thousands of new stop-loss orders were issued to keep soldiers from leaving the service after Gates ordered combat tours extended from 12 to 15 months last spring. The Army has resorted to involuntary extensions of soldiers' enlistment terms to prevent them from leaving immediately before a combat tour or in the middle of a deployment.