The United States Department of War, also called the War Department, 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.
The Cabinet of the United States is part of the executive branch of the U.S. government that normally acts as an advisory body to the President of the United States.
The United States Army is the largest branch of the United States Armed Forces and performs land-based military operations.
US in Vietnam War: "Why Vietnam?" 1965 United States Department of Defense, President Lyndon Johnson by Jeff Quitney
The Secretary of War, a civilian with such responsibilities as finance and purchases and a minor role in directing military affairs, headed the War Department throughout its existence.
Returning Soldiers: It's Your America 1945 US Army Training Film; World War II by Jeff Quitney
The War Department existed from August 7, 1789 until September 18, 1947, when it split into Department of the Army and Department of the Air Force and joined the Department of the Navy as part of the new joint National Military Establishment, renamed the United States Department of Defense in 1949.
The Department of Defense is an executive branch department of the federal government of the United States charged with coordinating and supervising all agencies and functions of the government concerned directly with national security and the United States Armed Forces.