U.S. Defense Secretary Robert Gates is retiring Thursday after leading the Pentagon for more than four years during key parts of the country's wars in Iraq and Afghanistan.

He took office in late-2006 under then-president George W. Bush, replacing former defense secretary Donald Rumsfeld, and oversaw a troop surge in Iraq that analysts say helped turned the campaign around.

Gates stayed on after the election of President Barack Obama, who ordered a similar surge of additional troops for Afghanistan. The defense chief leaves as the U.S. prepares to drawn down its forces in Afghanistan, with 10,000 troops leaving this year and another 23,000 by September 2012.

In a farewell message to U.S. forces Wednesday, Gates thanked service members and their families, and said their dedication and courage has kept America safe. He said their efforts brought the war in Iraq to a successful end and are “turning the tide” in Afghanistan.

Leon Panetta, who has been serving as the head of the U.S. Central Intelligence Agency, will succeed Gates as defense secretary.

Panetta has said his first task at the Pentagon will be to ensure the U.S. prevails in the conflicts it is engaged in, including in Iraq and Afghanistan.