The Defense Department is responsible for organizing, training, and equipping the United States armed forces and for planning and executing military operations to defend the country and its allies. It sits atop the Army, Navy, Air Force, Marine Corps, and Space Force, with the Coast Guard shifting to Defense Department control only in wartime (it normally sits under Homeland Security).

The Secretary of Defense is the principal civilian adviser to the president on military matters and, together with the president, forms the National Command Authority that alone can order the use of nuclear weapons. The department is led day to day from the Pentagon in Arlington, Virginia, the largest low-rise office building in the world.

Beyond combat operations, the department runs one of the largest health care systems, logistics networks, and research operations in the world, funds the military's own university system through the service academies, and manages a global footprint of hundreds of bases and installations across every populated continent.