WASHINGTON (AP) -- Defense Secretary Chuck Hagel is resigning at a particularly tough time for U.S. foreign and defense policy, with one war ending, another just beginning and the Pentagon struggling with the prospect of deeper budget cuts ahead.
It also raises the prospect of policy shifts as President Barack Obama seeks to sign up his fourth Pentagon chief in six years.
During a White House ceremony Monday after Hagel had submitted his resignation, Obama said he and Hagel agreed it was an "appropriate time for him to complete his service." Neither the president nor Hagel cited specific reasons for the change. Hagel aides said he had initiated private talks with the president in late October but was not leaving over policy conflicts.
Hagel, 68, never broke through the White House's notoriously insular national security team. Officials privately denigrated his ability to publicly communicate administration policy and more recently questioned his capacity to oversee new military campaigns against the Islamic State group in Iraq and Syria.
Hagel is the first high-level member of Obama's national security team to step down after both a disastrous midterm election for the president's party and persistent criticism about the administration's policies in the Middle East and elsewhere in the world. It's unclear whether Hagel's forced resignation signals the start of a broader shake-up of the president's team.
Obama said Hagel agreed to stay on the job until his successor has been confirmed by the Senate next year.
The timing sets up a potential confirmation fight. Republicans, who are about to take control of the Senate, have been deeply critical of the president's foreign policy.
Among leading contenders to replace Hagel is Michele Flournoy, a Democrat who served as the Pentagon's policy chief from 2009-12 and, after leaving, provided foreign policy advice to Obama's 2012 re-election campaign. 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 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.
Also mentioned as a possible successor is Ashton Carter, who served as deputy defense secretary from 2011-13.
With Hagel's ouster, Obama will be the first president since Harry Truman to have four defense secretaries. Hagel's predecessors, Robert Gates and Leon Panetta, complained after leaving the administration about White House micromanagement and political interference in policy decisions.
Rep. Howard "Buck" McKeon, the chairman of the House Armed Services Committee, suggested Obama consider his own role in his administration's foreign policy struggles rather than seeking another makeover at the Pentagon.
"When the president goes through three secretaries, he should ask, `Is it them, or is it me?'" said McKeon, R-Calif.
In some ways, the more mild-mannered Hagel was seen by the White House as 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. Others saw Hagel as slightly out of step with the White House.
"The focus has now shifted from budget cuts and (troop) withdrawals to new military action, especially in Syria and Iraq, and in the full course of that, he often hasn't seemed to be on the same page with the White House," said Stephen Biddle, a professor of political science at George Washington University's Elliott School of International Affairs.
Hagel's aides assert he is leaving at an appropriate juncture, after having brought to fruition this year several major reviews of problem areas for the Pentagon - most recently a plan for top-to-bottom changes in management of the nuclear force. He also took on reforms to the military justice system and to the military health system.
But the national security landscape looks far different than when Hagel was brought in to oversee the drawdown of the Afghanistan war and navigate the Pentagon through budget cuts. White House officials suggested the shift in emphasis was behind the need for a change in leadership.
The political skills Hagel showed as a two-term senator from Nebraska never fully translated to the Pentagon job, which is a complex mix of politics, public diplomacy, defense planning and management of a far-flung bureaucracy.
At times Hagel struggled to publicly articulate his views and the nuances of administration policy. Although he often visited military bases, he seemed reluctant to use his Vietnam combat experience as a way to connect. He was the first enlisted combat veteran to serve as Pentagon chief.
Hagel joined the Army at age 21. After completing training, he volunteered to fight in Vietnam even though the Army intended to send him to Germany as part of a classified project involving a new shoulder-fired missile.
"All my friends thought I was out of my mind" to insist on Vietnam, he said in a 2002 interview with a Library of Congress veterans history project. "Nonetheless, I just felt it was the right thing to do. A war was going on. They needed their best people, and I didn't want to be in Germany when there was a war going on in Vietnam."