the United States and on August 28, 1861, he was commissioned as an officer and given command of the 55th New York Volunteer Infantry, the predominantly French-immigrant regiment known as the Gardes de Lafayette. He and his regiment were attached to Peck's Brigade of Couch's Division, Keyes's IV Corps of the Army of the Potomac in September 1861. They took part in the 1862 Peninsula Campaign, seeing first combat on May 5, 1862, at the Battle of Williamsburg. Soon after, Trobriand was debilitated with a malady diagnosed as "swamp fever", missed the remainder of the campaign, and was unable to return to duty until July. His regiment's next engagement, part of the brigade of Brig. Gen. J. H. Hobart Ward, III Corps of the Army of the Potomac, was at the Battle of Fredericksburg. They were held in reserve and escaped the terrible bloodshed of the Union defeat. In December 1862, the 55th and 38th New York were merged, and Trobriand became the colonel of the now-named 38th. He led his new regiment at the Battle of Chancellorsville in May 1863, but was not heavily engaged. After the III Corps was reorganized following its severe casualties at Chancellorsville, Trobriand was given command of a new brigade. Trobriand's military career is best known for the Battle of Gettysburg, where he first saw significant action. He arrived on the second day of battle, July 2, 1863, and took up positions in the area known as the Wheatfield. His brigade put up a spirited defense against powerful assaults by Confederate Maj. Gen. John Bell Hood's division, particularly a Georgia brigade under Brig. Gen. George T. Anderson and a South Carolina brigade under Brig. Gen. Joseph B. Kershaw. They successfully held out until relieved by units of Maj. Gen. John C. Caldwell's division of the II Corps, but it came at a terrible price—every third man in Trobriand's brigade was a casualty. After the battle, his division commander, Maj. Gen. David B. Birney, wrote: Despite the recommendation and his excellent performance at Gettysburg, Col. Trobriand did not receive a promotion to brigadier general until his appointment to that grade by President Abraham Lincoln on April 10, 1864, to rank from January 5, 1864, after the U.S. Senate had confirmed the appointment on April 7, 1864. He finally assumed command of a brigade to match his rank when Brig. Gen. J. H. Hobart Ward was