The United States Department of War, also called the War Department (and occasionally War Office in the early years), was the United States Cabinet department originally responsible for the operation and maintenance of the United States Army, also bearing responsibility for naval affairs until the establishment of the Navy Department in 1798, and for most land-based air forces until the creation of the Department of the Air Force on September 18, 1947. **Source**: [United States Department of War](https://en.wikipedia.org/wiki/United_States_Department_of_War) on Wikipedia.