This department, the Department of War was created shortly after the end of the American Revolutionary War in 1789 and it disbanded in 1947 which is shortly after World War II. This department was responsible for the military at the time it was the army and navy. Later, it became responsible for the Air Force. The person in charge of this department is the Secretary of War.
The main reason that the Department of War was dissolved is due to the size of how the different military branches grew. In 1947, the Department of War was done away with and three other departments were created in its place. These were the Department of the Army, Department of the Air Force and the Department of the Navy.