Karl W. Eikenberry, a retired Army lieutenant general, was the United States commander in Afghanistan from 2005 to 2007 and the ambassador there from 2009 to 2011. He is a fellow at Stanford, where David M. Kennedy is an emeritus professor of history. They are, respectively, a contributor to and the editor of “The Modern American Military.”STANFORD, Calif. — AFTER fighting two wars in nearly 12 years, the United States military is at a turning point. So are the American people. The armed forces must rethink their mission. Though the nation has entered an era of fiscal constraint, and though President Obama last week effectively declared an end to the “global war on terror” that began on Sept. 11, 2001, the military remains determined to increase the gap between its war-fighting capabilities and those of any potential enemies. But the greatest challenge to our military is not from a foreign enemy — it’s the widening gap between the American people and their armed forces.