For 158 years, the United States had a Department of War, but that agency was much different from the modern Department of War, which was created after World War II to merge all four branches of the military under one executive department.
When was the Department of War created?
On August 7, 1789, during the very first session of Congress, President George Washington signed legislation creating the Department of War. Also known as the War Office (a term borrowed from the British), the Department of War was primarily in charge of the Army. The Navy got its own Cabinet-level department in 1798, which also oversaw the Marines.
The first secretary of war was Henry Knox, a close aide to Washington during the Revolutionary War who was already serving as secretary at war under the Articles of Confederation.
The new Department of War replaced the Board of War and Ordnance, which Washington established in 1776 to administer the military during the Revolution.
What’s the significance of the National Security Act of 1947?
World War II required unprecedented levels of coordination across the different branches of the U.S. military to fight on land, sea and air. After the war—as the Soviet Union emerged as a threat—President Harry Truman signed the National Security Act of 1947, a major restructuring of U.S. military administration.
First, the act replaced the Department of War with the Department of the Army. Next, it created the U.S. Air Force as its own branch, separate from the Army. Then all four branches of the military were merged under a single organization called the National Military Establishment.
Under the 1947 law, the head of the National Military Establishment (NME) was the secretary of defense, a new Cabinet-level position. The defense secretary had to be a civilian or retired from the military for 10 years (later lowered to seven years). The first secretary of defense was James Forrestal, former secretary of the Navy.
The National Security Act of 1947 also created other fixtures of U.S. national security, including the Joint Chiefs of Staff, the Central Intelligence Agency and the National Security Council.
When did it become the Department of Defense?
The National Military Establishment didn’t last long. Just two years later, Truman signed a series of amendments to the National Security Act that replaced it with the Department of Defense.
The problem with the National Military Establishment was that the secretary of defense, who was technically in charge, didn’t have authority to resolve disputes between the branches of the military. That’s because the secretaries of the Army, Navy and Air Force still had seats in the president’s Cabinet.
The 1949 amendments to the National Security Act created the Department of Defense as a new executive department. On top of that, the secretaries of the Army, Navy and Air Force were removed from the Cabinet, and the secretary of defense was given total “direction, authority and control” over the new department, according to Truman.
The changes to the National Security Act included the creation of a Chairman of the Joint Chiefs of Staff, who would advise both the president and the secretary of defense directly on military matters. The first chairman was General Omar Bradley, appointed in 1949.
In 2025, President Donald Trump issued an executive order restoring the use of Department of War as a "secondary title" for the Department of Defense. Only Congress can approve a permanent name change.
