Mark Esper is the acting secretary of defense for the U.S. Department of Defense. President Donald Trump (R) nominated him to this position on June 18, 2019, after former Acting Secretary of Defense Patrick Shanahan withdrew his name from consideration.
Prior to his nomination, Esper was the secretary of the United States Army. He served on active duty for over a decade, after which he served in the National Guard and Army Reserve.
The secretary of defense oversees the military departments, the chairman of the Joint Chiefs of Staff, and the unified commands. According to the Department of Defense's website, "The military departments train and equip the military forces. The Chairman plans and coordinates military deployments and operations. The unified commands conduct the military operations."