Buchanan received his appointment to United States Military Academy at West Point during Adams' administration and graduated in 1830. He was soon assigned to the 4th U.S. Infantry as a brevet second lieutenant. His assignments included service in the Black Hawk War (he commanded gunboats during the Battle of Bad Axe) and against the Seminoles, as well as in the removal of the Cherokees to the Indian Territory. He was promoted to captain during his service in Florida.