On April 17, 1861, just five days after the firing on Fort Sumter, he enlisted as a private in the 7th New York Militia, one of the first regiments to come to the defense of Washington, D.C. at the outbreak of the Civil War. He later enlisted in 1861 at age 24 as a first lieutenant in 12th U.S. Infantry. He served in Brig. Gen. George Sykes's division of V Corps in the Seven Days Battles, being commended by his superiors for his conduct at the Battle of Gaines' Mill on June 27, 1862.