In 1861, at the beginning of the U.S. Civil War, he was Adjutant of the Third Artillery. Shortly thereafter, he resigned to join the staff of General Edwin Vose Sumner's as senior "aide-de-camp", achieving the rank of Major. He was a part of the Army of the Potomac which saw action in the Battles of Yorktown, Williamsburg, Fair Oaks, Seven Pines, Savage's Station, Glendale, Malvern Hill, Antietam, Fredericksburg (all of which took place in 1862), and Mine Run which lasted from November 1863 until December 1863.