US Defense Secretary Chuck Hagel has stepped down from his post, reportedly under pressure from the White House.Discussions among members of the National Security Council, and an itch from US President Barack Obama for a change in its makeup, led to Hagel’s resignation, officials said on Monday. The announcement was made by the president from the State Dining Room that morning.In the ceremony, attended by National Security Adviser Susan Rice, Vice President Joe Biden and other senior administration officials, Obama thanked Hagel for a busy year of service, for his “straight” talk in the Oval Office and for serving in a Democratic administration as a career Republican.“We come from different parties,” he said, “but taking this position, you sent a powerful message.”And yet Hagel’s exit, after a combative nomination process through Congress and only a year at the Pentagon, is the third such resignation of a defense secretary during Obama’s presidency.His fourth, yet to be named, would provide the president with more defense secretaries than any other modern president.Hagel was brought in to the Pentagon to manage the winding down of the US war in Afghanistan and shrinking defense budgets. What he helmed, instead, was the launch of a war against Islamic State, a terrorist organization that has amassed significant wealth, recruits and territory across Iraq and Syria over Hagel’s tenure.Hagel raised questions about Obama’s strategy toward Syria in a two-page internal policy memo that leaked this fall. In it, he warned that Obama’s policy was in jeopardy due to its failure to clarify its intentions toward Syrian President Bashar Assad.“This is beyond anything that we’ve seen,” Hagel said in August of Islamic State, after the group began beheading American hostages on video. “We must prepare for everything.”Officials said Obama wanted fresh leadership during the final two years of his administration.“What I can tell you is there are no policy differences in the background of this decision,” a senior US defense official said. “The secretary is not resigning in protest and he’s not being fired.”Top potential candidates to replace Hagel include Michèle Flournoy, a former under secretary of defense, and Ashton Carter, a former deputy secretary of defense, who were rumored to be contenders for Hagel’s job before he was named. Senator Jack Reed (D-Rhode Island) is another possible contender.Hagel, who was the only enlisted combat veteran to serve as defense secretary, ran into a wave of opposition when Obama, a Democrat, nominated him.Hagel’s two predecessors under Obama, Robert Gates and Leon Panetta, both served longer terms and both wrote memoirs, after leaving the Pentagon, critical of the president’s leadership.Obama’s choice for Hagel’s successor will likely face fierce questioning from a new Republican Senate and its expected new chairman of the Armed Services Committee, John McCain (R-Arizona).“I know that Chuck was frustrated with aspects of the administration’s national security policy and decision-making process,” McCain, a frequent critic of Obama’s foreign policy, said in a statement on Monday. “His predecessors have spoken about the excessive micromanagement they faced from the White House and how that made it more difficult to do their jobs successfully.”“Ultimately,” he continued, “the president needs to realize that the real source of his current failures on national security more often lie with his administration’s misguided policies and the role played by his White House in devising and implementing them.”Reuters contributed to this report.