Defense Secretary Chuck Hagel is quitting at a messy moment for U.S. foreign policy, with one war ending and another just beginning.

His forced departure reflects White House concerns about his effectiveness and raises the prospect of policy shifts as President Barack Obama looks to sign up his fourth Pentagon chief in six years.

Hagel said Monday he will stay until a successor is confirmed by the Senate.

Among the leading candidates is Michele Flournoy, a former Pentagon policy chief who would be the first woman to serve as defense secretary.

In a White House announcement Monday, Obama said he and Hagel agreed it was an "appropriate time for him to complete his service." Hagel is the first enlisted combat veteran to head the Pentagon.