Fort Leavenworth , U.S. military post, 6,000 acres (2,430 hectares), on the Missouri River, NE Kans., NW of Leavenworth; est. 1827 by Col. Henry Leavenworth to protect travelers on the Santa Fe Trail . The oldest U.S. military prison (est. 1874), and the U.S. Army Command and General Staff College are at the fort. The Buffalo Soldier Monument honors the post-Civil War African-American regiments that served in the West (see buffalo soldiers ).