Less than two months after graduation from West Point, he became the first regular army officer to be wounded in battle while serving as captain of the Fifth New York Infantry at Big Bethel, Virginia. He returned to service as lieutenant colonel of the 2nd New York Cavalry. Quickly, he rose through the ranks to become a brigadier general in June 1863. He served in the Battle for Beverly's Ford, Stoneman's raid, and the Battle of Gettysburg. In April 1864 after his attempt to raid the prison camp at Richmond, he was sent to take charge of a division of Sherman's cavalry in the West. During the Atlanta campaign he was badly wounded at Resaca. He was forced to sit out the rest of the war guarding Sherman's communications and commanding the Union forces during the skirmishes at Lovejoy's Station and Buck Head Creek. In June 1865, after the end of the war, he was brevetted major general. He resigned both of his commissions in December 1865.