The Department of Defense was birthed during the aftermath of World War II. In 1947, the National Security Act joined the Department of War and the Department of the Navy into a single entity. During the war, the government had constructed a new headquarters for its military department, the Pentagon, which became the home of the new Department of Defense. Under the new organization, the War Department split into the Department of the Army and the Department of the Air Force. The National Security Act also formalized the creation of the Joint Chiefs of Staff, a group of the highest-ranking military officials from each service, who had been meeting throughout the war to coordinate their strategy.
Today, the Armed Forces are divided in two different ways, by service and by command. Three service departments - the Army, the Navy and the Air Force - organize members of the military according to the type of work they do and the arena in which they fight. (The Marine Corps falls under the Department of the Navy). Ten unified combatant commands organize forces from all the services into geographical or functional areas. Forces from the Army, Navy and Air Force all serve in Central Command, which has responsibility for the military's work in Iraq and Afghanistan, for instance.