Short answers, pulled from the story.
The United States Department of War existed for 158 years, from the 7th of August 1789, to the 18th of September 1947. It was dissolved by the National Security Act of 1947, which split it into the Department of the Army and the Department of the Air Force.
Retired senior General Henry Knox was the first United States secretary of war. He took the position when the War Department was reestablished in 1789 under President George Washington.
After World War II, the American government and other governments around the world decided to stop using the word "War" when referring to the civilian leadership of their militaries. The National Security Act of 1947 replaced the War Department with the Department of the Army and the Department of the Air Force, later grouped under the Department of Defense in 1949.
The Pentagon was completed in 1943, and the secretary of war moved in at that point. Congress had authorized funding for the Pentagon on the 28th of July 1941, in response to the War Department being scattered across the overcrowded Munitions Building and numerous other offices around the Washington, D.C., area.
The Bureau of Indian Affairs was created by Secretary of War John C. Calhoun in 1824 as the main agency within the War Department for addressing issues regarding Native Americans. Congress transferred it to the newly founded Department of the Interior in 1849.
Elihu Root, serving from 1899 to 1904, established the United States Army War College and the General Staff, enlarged West Point, reformed officer promotion procedures, and introduced the practice of rotating officers between staff and line positions. He also developed the procedures for returning Cuba to Cuban governance and wrote the charter of government for the Philippines.