In May 1861 he returned to U.S. military service as 1st Lieutenant in the 1st U.S. Cavalry Regiment. He was part of the regular Cavalry service until he was appointed Colonel of the 1st Massachusetts Volunteers Infantry Regiment. In this capacity he fought at Fredericksburg, Chancellorsville, Gettysburg, Mine Run, Wilderness and Spotsylvania. He won brevet promotions to major in the U.S. Army for Chancellorsville and to lieutenant colonel for Gettysburg. He was mustered out of the volunteers service on May 28, 1864.