Among the leading contenders to replace Hagel is Michele Flournoy, who served as the Pentagon‘s policy chief for the first three years of Obama’s presidency. Flournoy, who would be the first woman to head the Pentagon, is now chief executive officer of the Center for a New American Security, a think tank that she co-founded.

Flournoy is said to be interested in the top Pentagon job but seeking assurances from the White House that she would be given greater latitude in policymaking than Hagel. Flournoy is also considered a possible defense secretary for Hillary Rodham Clinton if Clinton should win the presidency in 2016.

Others mentioned as possible replacements include Ashton Carter, the former deputy defense secretary, and Robert Work, who currently holds that post.

With Hagel‘s departure, Obama will be the first president since Harry Truman to have four defense secretaries. Hagel’s two predecessors, Robert Gates and Leon Panetta, complained after leaving the administration about White House micromanagement and political interference in policy decisions.

Rep. Buck McKeon, chair of the House Armed Services Committee, suggested Obama consider his own role in his administration‘s foreign policy struggles rather than seeking another changeover at the Pentagon.

``When the president goes through three secretaries, he should ask, `Is it them or is it me?’‘’ said McKeon, a Republican.

Hagel has had his own frustrations with the White House. In recent weeks, he sent a letter to National Security Adviser Susan Rice in which he said Obama needed to articulate a clearer view of the administration‘s approach to dealing with Syrian President Bashar Assad. The letter is said to have angered White House officials.

In some ways, Hagel was seen as an attempt by the White House to install a Pentagon chief who would be less likely than Gates and Panetta to pitch policy fights with the West Wing. Some foreign policy experts noted the irony in the White House ousting a defense secretary who largely played the role the president appeared to have been seeking.

``The White House picked him because they wanted somebody they could control and would be a policy nonentity and they got a policy nonentity,’‘ said Rosa Brooks, who served at the Pentagon during Obama’s first term. ``It seems unfair to make him a fall guy for White House policy failures.‘’

The timing of Hagel‘s departure sets up a potential confirmation fight in the Senate. Republicans, who will take control of the body next month, have been deeply critical of the president’s foreign policy. Hagel has agreed to remain until a successor is confirmed by the Senate. (AP)