At the outbreak of the American Civil War, Moore was appointed from Illinois as a First Lieutenant in the 13th Wisconsin Volunteer Infantry in October 1861.
During the Gettysburg Campaign, Moore served on the staff of Major General Daniel Sickles, commander of the 3rd Corps of the Union Army of the Potomac.
He was present in the Peach Orchard at the Battle of Gettysburg on July 2, 1863, when Sickles lost his right leg from a Confederate artillery shell.
In 1864, Moore served on a tour of inspection before being appointed to the staff of Brigadier General Alfred Torbert in the Shenandoah Valley.
Alexander Moore died on September 30, 1910, in Washington, D.C., and was buried with full military honors beside his wife in Arlington National Cemetery.