US Defence Secretary Chuck Hagel has resigned, ending nearly two years in the Pentagon's top job.
White House officials say the 68-year-old submitted his resignation letter after lengthy discussions with the President Barack Obama that began in October.
Mr Obama announced the resignation at a White House event with Mr Hagel at his side. The former Republican senator and Vietnam war veteran will continue as defence secretary until a replacement is found.
Mr Hagel was appointed less than two years ago as the US president pushed his signature programme of winding up wars in Afghanistan and Iraq, a process being changed this year with US re-engagement in Iraq and greater military cooperation with Kabul.
Mr Obama said at the White House event that Hagel had always been candid with his advice and had "always given it to me straight."
The defence secretary raised questions about Mr Obama's strategy toward Syria in a two-page internal policy memo that leaked last month. In it, he warned that Mr Obama's policy was in jeopardy due to its failure to clarify its intentions toward Syrian President Bashar al-Assad.
President Obama has insisted that the United States can go after Islamic State militants without addressing the Syrian president, who the United States would like to leave power.
Officials said Mr 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 defence official told Reuters. "The secretary is not resigning in protest and he's not being 'fired'," the official said.
Top potential candidates to replace Mr Hagel include Michele Flournoy, a former under secretary of defence, and Ashton Carter, a former deputy secretary of defence, who were rumoured to be contenders for Hagel's job before he was named. Senator Jack Reed, Democrat of Rhode Island, is another possible contender.
Mr Hagel, who was the only enlisted combat veteran to serve as defence secretary, ran into a wave of opposition when Mr Obama, a Democrat, nominated him.
Republicans objected because Mr Hagel opposed the 2007 Iraq war 'surge' of troops, which eventually helped defeat al-Qaeda and other militants and opened the way for a US troop withdrawal.
He had also upset many in his party by endorsing Obama in his presidential race against Republican Senator John McCain in 2008.