[5] Retired senior General Henry Knox, then in civilian life, served as the first United States Secretary of War.

[8] The Department of War was also responsible for overseeing interactions with Native Americans in its early years.

[10] The United States Military Academy at West Point and the Army Corps of Engineers were established in 1802.

[12] To accommodate this expansion, sub-departments were created within the department, with each one led by a general staff officer.

[13] These sub-departments were reformed into a modern system of bureaus by Secretary of War John C. Calhoun in 1818.

[18] During the Reconstruction era, this bureau played a major role in supporting the new Republican governments in the southern states.

Concerned about the new territories acquired after the Spanish–American War, Root worked out the procedures for turning Cuba over to the Cubans, wrote the charter of government for the Philippines, and eliminated tariffs on goods imported to the United States from Puerto Rico.

Indeed, Secretary Taft exercised little power; President Theodore Roosevelt made the major decisions.

In 1911, Secretary Henry L. Stimson and Major General Leonard Wood, his chief of staff, revived the Root reforms.

The general staff assisted them in their efforts to rationalize the organization of the army along modern lines and in supervising the bureaus.

[22] The Congress reversed these changes in support of the bureaus and in the National Defense Act of 1916 reduced the size and functions of the general staff to few members before America entered World War I on April 6, 1917.

President Woodrow Wilson supported Secretary of War Newton D. Baker, who opposed efforts to control the bureaus and war industry until competition for limited supplies almost paralyzed industry and transportation, especially in the North.

Assisted by industrial advisers, they reorganized the supply system of the army and practically wiped out the bureaus as quasi-independent agencies.

In the early years, between 1797 and 1800, the Department of War was headquartered in Philadelphia; it moved with the other federal agencies to the new national capital at Washington, D.C., in 1800.

In 1820, headquarters moved into a building at 17th Street and Pennsylvania Avenue NW, adjacent to the Executive Mansion, part of a complex of four matching brick Georgian/Federal style buildings for Cabinet departments with War in the northwest, Navy in the southwest and to the other side: State to the northeast and Treasury in the southeast.

In August 1939, Secretary of War Harry H. Woodring and Acting Chief of Staff of the Army George C. Marshall moved their offices into the Munitions Building, a temporary structure built on the National Mall during World War I.