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.

Three developments in recent decades have widened this chasm. First and most basic was the decision in 1973, at the end of combat operations in Vietnam, to depart from the tradition of the citizen-soldier by ending conscription and establishing a large, professional, all-volunteer force to maintain the global commitments we have assumed since World War II. In 1776, Samuel Adams warned of the dangers inherent in such an arrangement: “A standing Army, however necessary it may be at some times, is always dangerous to the Liberties of the People. Soldiers are apt to consider themselves as a Body distinct from the rest of the Citizens.”

For nearly two generations, no American has been obligated to join up, and few do. Less than 0.5 percent of the population serves in the armed forces, compared with more than 12 percent during World War II. Even fewer of the privileged and powerful shoulder arms. In 1975, 70 percent of members of Congress had some military service; today, just 20 percent do, and only a handful of their children are in uniform.

In sharp contrast, so many officers have sons and daughters serving that they speak, with pride and anxiety, about war as a “family business.” Here are the makings of a self-perpetuating military caste, sharply segregated from the larger society and with its enlisted ranks disproportionately recruited from the disadvantaged. History suggests that such scenarios don’t end well.