The United States armed services belonged to separate cabinet-level departments during WW II. The original Departments of War and Navy were now joined by the burgeoning Air Force, which was climbing out into independence from control by the ground-based Army. To streamline the chain of command, the Department of Air Force was split off from the Army, with each given their own secretary, and a new organization called National Military Establishment was founded, to be headed up by the new cabinet-level post, the Secretary of Defense.

On this day, August 10, in 1949 President Truman signed an amendment to the National Security Act of 1947 that created the different departments, renaming the NME to the Department of Defense. The amendment also addressed the chain of command problem brought on by the NSA Act.

The secretaries of both the Army and Air Force enjoyed quasi-cabinet level power, and as a result James Forrestal, the first appointed Secretary of Defense, found exercising authority over them next to impossible. The amendment streamlined the chain of command, putting the leaders of all armed serviced under the direction of the Secretary of Defense.