The defense chief submitted his resignation letter to US President Barack Obama early Monday.
Defense Secretary Chuck Hagel speaks during a news conference at the Pentagon, Washington, Nov. 14, 2014.
Defense Secretary Chuck Hagel talks to marines at a training range as he observes an experimental infantry unit that is going to help assess which combat jobs can be opened to women in 2016, Camp Lejeune, North Carolina, Nov. 18, 2014.
Defense Secretary Chuck Hagel, accompanied by Joint Chiefs Chairman Gen. Martin Dempsey, testifies on Capitol Hill before the House Armed Services committee hearing on the Islamic State militants, Washington, Nov. 13, 2014.
President Barack Obama, next to Defense Secretary Chuck Hagel, speaks to the media about Ebola during a meeting in the Cabinet Room of the White House in Washington, Oct. 15, 2014.