Outgoing U.S. Defense Secretary Robert Gates is the only Pentagon chief in U.S. history to be asked to remain in the post by a newly-elected president.

Gates was sworn in under President Barack Obama's predecessor, George W. Bush, in 2006, at a crucial point in the war in Iraq.

He oversaw a troop surge in Iraq that analysts say helped turned the campaign around, and later presided over a similar surge of troops for Afghanistan.

The 67-year-old Gates retires after four decades in public service.

He joined the U.S. Central Intelligence Agency in 1966, spending nearly three decades as an intelligence professional. He is the only career officer in CIA history to rise from entry-level employee to director, heading the agency in the early 1990s.

Gates also served nearly nine years on the White House National Security Council, and was deputy national security adviser for President George H.W. Bush.

Before leading the Pentagon, Gates was the president of one of the nation's largest universities, Texas A&M — home to the George Bush School of Government and Public Service.