The United States Department of War, also called the War Office, was the department of the United States government's executive branch responsible for the operation and maintenance of land (and later air) forces from 1789 until September 18, 1947, when it became part of the National Military Establishment, renamed on August 10, 1949 the Department of Defense. The War Department also had responsibility for the young nation's naval affairs until the establishment of the Navy Department in 1798.