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The US Department of War existed with this name between 1789 and 1947. After WWII, it was splitted in 3: The Department of the Air Force, the Department of the Army and the Department of the Navy. The three of them formed the joint National Military Establishment (NME). The final name change took place in 1949 and established the current denomination: United States Department of Defense.
The Department of War started with the aim of maintaining and operating the US military forces. Nowadays, the department of defense is established as one of the executive governmental branches, and its function is to coordinate and monitor all activities related to national security and the US Armed Forces.
The Department of War was established in 1789 to manage the US military. The department established an official army instead of relying on militias. The National Reorganization Act of 1947 changed this division of the cabinet by creating the Department of Defense. It oversees the nation's military actions. In 2002, the Department of Homeland Security was formed to focus on safety within the nation's borders.