Short answers, pulled from the story.
The United States Department of War was reestablished on the 7th of August 1789 by Congress under President George Washington. Henry Knox served as the first secretary of war while maintaining his civilian status.
The War Department building along with its records and files was consumed by fire on the 8th of November 1800. The department had been headquartered in Philadelphia between 1797 and 1800 before moving to Washington D.C.
Secretary Elihu Root enlarged the United States Military Academy at West Point New York and established the United States Army War College and General Staff. He changed promotion procedures and organized schools for special branches of service while devising a principle of rotating officers from staff to line.
The National Security Act of 1947 split the War Department into the Department of Army and Department of Air Force because independent services continually parried efforts to reestablish firm executive control over their operations. This act created the National Military Establishment which later became the Department of Defense in 1949.
The original 1820 structures were replaced by the State War Navy Building now known as the Old Executive Office Building named honor General President Dwight D. Eisenhower. The department moved offices to the Pentagon in Arlington Virginia after construction completed in 1943.