John Ryan spent nearly 14 years between 1862 and 1876 in the U.S. Army and took part in many of the most celebrated military encounters of that era. Initially a private with the 28th Massachusetts Volunteer Infantry Regiment of the Irish Brigade, he charged with his comrades at Second Bull Run, survived the fiery plains beneath Marye's Heights at Fredericksburg and in the Wheatfield at Gettysburg, and was wounded three times on his 19th birthday, August 25, 1864, during the Battle of Reams Station, Virginia, in the Petersburg Campaign.
In November 1866, Ryan enlisted again in the Army, this time in the 7th U.S. Cavalry in which he would serve for ten years alongside its legendary commander, George Armstrong Custer. With Custer, Ryan charged the Cheyenne village of Black Kettle in November 1868 and took part in the Yellowstone Expedition of 1873 and the Black Hills Expedition of 1874. But his last battle at the Little Big Horn in June 1876 would be what he would talk about and write about for 50 years after his final discharge. First Sergeant with Company M, Ryan fought heroically in the valley and on the bluffs with the battalion under Major Marcus A. Reno.