The United States Department of War was the military department of the United States government's executive branch from 1789 until 1949, when it became part of the United States Department of Defense. It was headed by the United States Secretary of War. It was also known as the War Office.

In 1949 the War Office was renamed the United States Department of the Army and became a component of the Department of Defense. The United States Secretary of the Army is in charge of the administrative offices necessary for the Army's operations. The highest-ranking Army officer is the Army Chief of Staff who is assisted by the Vice-chief. The only difference today is that the Army Air Force was separated and formed into the US Air Force under the newly formed Department of the Air Force.