WASHINGTON—The bulk of U.S. military forces will depart Haiti on Tuesday, leaving United Nations forces and civilian groups to help the country rebuild its devastated capital in the wake of January's deadly earthquake.

The departure, which was agreed upon by the U.S. and Haitian governments and long anticipated, signals that "we have reached a basic level of sustainment in Haiti," said State Department spokesman P.J. Crowley in an interview on Monday. "The military played an essential short-term role, but now this is getting...