Participating in campaigns against the Comanche while stationed in Fort Inge, Arnold held a distinguished service record during the American Civil War. Promoted to first lieutenant in April 1861, he served as adjutant of the 5th Cavalry Regiment. He was cited "for gallant and meritorious services" at Gaines' Mill and Todds Tavern, brevetted to captain and major after both engagements respectively. He was awarded the Medal of Honor for his actions during the Battle of Davenport Bridge leading his regiment in a cavalry charge against superior Confederate forces to rescue men under his command and preventing their capture.