Explanation:
Because it is recognized by many people
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 primary purpose of Franklin Roosevelt's WPA was "A. To help get America through the Great Depression" since it put thousands of Americans back to work on public projects.