WASHINGTON » Under pressure from President Barack Obama, Defense Secretary Chuck Hagel submitted his resignation Monday amid White House concerns about his effectiveness and broader criticism from outside about the administration’s Middle East crisis management.

The president said he and Hagel had determined it was an "appropriate time for him to complete his service."

Hagel, a former Republican senator, never broke through the White House’s notably insular national security team. Officials privately griped about his ability to publicly communicate administration policy and more recently questioned whether he had the 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 in the wake of both a disastrous midterm election for the president’s party and persistent criticism about the administration’s policies in the Middle East and elsewhere. It’s unclear whether Hagel’s forced resignation signals the start of a broader shake-up of the president’s team; White House officials said it was possible there could be more departures.

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, R-Calif.

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 submitted his resignation letter to Obama on Monday morning after what his advisers said were a series of private discussions about his future that he initiated with Obama last month. The 68-year-old has agreed to remain in office until a successor is confirmed by the Senate.

Hagel’s aides assert that he is leaving at an appropriate juncture. The aides were vague about what persuaded him to quit, suggesting that he ultimately agreed with Obama that the Pentagon needed a new leadership focus for the coming two years.

Hagel has steered the Pentagon through a major review of nuclear weapons management, as well as reforms to the military justice system and to the military health system. But his departure also coincides with a period of great uncertainty over the course of the administration’s campaign to defeat the Islamic State group, as well as worry over Russia’s actions in Ukraine, and further defense budget cuts.

Indeed, the current landscape looks far different from when Hagel was brought in to oversee the drawdown of the Afghanistan war and navigate the Pentagon through the cutbacks. White House officials suggested the shift in emphasis was behind the need for a change in leadership.

Hagel served as senator from Nebraska and became a critic of U.S. involvement in Iraq. He forged a strong personal relationship with Obama in the Senate and they made several overseas trips together, including the high-profile visit Obama made in the final months before the 2008 presidential campaign. Hagel also carved out a reputation as an independent thinker and blunt speaker, and Obama said he came to admire his courage and willingness to speak his mind.

Hagel was the first enlisted combat military veteran to become secretary of defense. He served in the Vietnam War and received two Purple Hearts.

As he announced his resignation Monday, Hagel said it was "the greatest privilege of my life to lead and, most important, to serve with the men and women of the Defense Department and support their families."

Associated Press writers Donna Cassata and Nedra Pickler contributed to this story.