The United States Assistant Secretary of War was the second-ranking official within the American Department of War from 1861 to 1867, from 1882 to 1883, and from 1890 to 1940. According to the Military Laws of the United States, "The act of August 5, 1882 authorizing the appointment of an Assistant Secretary of War was repealed by the act of July 7, 1884 (23 Stat L., 331) the power conferred by the act of August 5, 1882 never having been exercised," indicating that the post was not filled between 1882 and 1883 (p. 45, footnote 2).
In 1940, the new position of United States Under Secretary of War replaced this position as the number-two office in the department. Assistant Secretary Robert P. Patterson became the first Under Secretary.
The office continued to exercise administrative duties until the department's end in 1947, when the United States Department of Defense