 On February 8, 1864, he was promoted to colonel of the 1st U.S. Infantry Regiment through seniority.For his service at the Battle of Malvern Hill, on April 10, 1866, President Andrew Johnson nominated Buchanan for appointment to the grade of brevet brigadier general, U.S. Army, to rank from March 13, 1865, and the U.S. Senate confirmed the appointment on May 4, 1866.In recognition of Buchanan's service at the Battle of Second Bull Run and the Battle of Fredericksburg, on June 30, 1866, President Andrew Johnson nominated Buchanan for appointment to the grade of brevet major general, U.S. Army, to rank from March 13, 1865, and the U.S. Senate confirmed the appointment on July 25, 1866.