In 1858 he went to New York, where he first taught Italian and French. In February 1861 he married Mary Isabel Reid, the daughter of war hero Commodore Samuel Chester Reid. He then founded a private military school for officers, where in six months he trained over seven hundred students. Since 1862, he took part in the American Civil War as colonel of the 4th New York Cavalry Regiment, serving under the name Louis P. di Cesnola. Accused of sending stolen property North, Cesnola was threatened with dishonorable dismissal from the army. However, at the Battle of Aldie (June 1863), Colonel di Cesnola was wounded and taken prisoner. He received a Medal of Honor for his efforts during the battle. He was released from Libby Prison early in 1864 when the Union Agent for Prisoner Exchange offered a personal friend of Jefferson Davis as barter. He served in the Wilderness and Petersburg campaigns (1864–65) as a commander of a cavalry brigade but was not promoted to brigadier general. Although he was nominated for appointment to the brevet grade of brigadier general to rank from March 13, 1865 after the end of the war, the U.S. Senate never confirmed his appointment (contrary to the inscription on his grave stone).