The secretary of war was a member of the U.S. president's Cabinet, beginning with George Washington's administration.

When Washington was inaugurated as the first President under the Constitution, he appointed Knox to continue serving as Secretary of War.

In 1798, the secretary of the Navy was created by statute, and the scope of responsibility for this office was reduced to the affairs of the United States Army.

The office of Secretary at War was modeled upon Great Britain's secretary at war, who was William Barrington, 2nd Viscount Barrington, at the time of the American Revolution.

The office of Secretary at War was meant to replace both the commander-in-chief and the Board of War, and like the president of the board, the secretary wore no special insignia.