Fort Leavenworth (lĕvˈənwûrthˌ) [key], 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).