The Secretary of Defense in the United States of America deals with military matters and was previously know as Secretary of War. Under George W. Bush that could mean defending ourselves against non-existent Weapons of Mass Destruction in Iraq.
The previous Secretary of Defense was Leon Panetta. Panetta stepped down for Obama's second term, and Obama nominated Chuck Hagel to be his replacement. Unfortunately, Republicans fillibustered the nomination, the first time this has ever happened, even though Hagel is also a Republican (but he does have liberal views on foreign policy). John McCain justified this by pointing out Hagel was mean to George W. Bush. Eventually, they gave up, and Hagel got the job. Currently Jim Mattis is Secretary of Defense.