Charles Devens and George Frisbie Hoar in Worcester, Massachusetts. Military service 15th Massachusetts Infantry As tensions between the North and South increased over the winter of 1860–1861, Walker equipped himself and began drilling with Major Devens' 3rd Battalion of Rifles in Worcester and New York. Despite his older brother Robert serving in the 34th Massachusetts Infantry, his father objected to his youngest son mobilizing with the first wave of volunteers. Walker returned to Worcester but began to lobby William Schouler and Governor John Andrew to grant him a commission as a second lieutenant under Devens' command of the 15th Massachusetts. Following his 21st birthday and the First Battle of Bull Run in July 1861, Walker secured the consent of his father to join the war effort as well as assurances by Devens that he would receive an officer's commission. However, the lieutenancy never materialized and Devens instead offered Walker an appointment as a sergeant major, which he assumed on August 1, 1861, after re-tailoring his previously ordered lieutenant's uniform to reflect his enlisted status. However, by September 14, 1861, Walker had been recommended by Devens and reassigned to Brig. Gen. Darius N. Couch as assistant adjutant general and promoted to captain. Walker remained in Washington, D.C., over the winter of 1861–1862 and did not see combat until May 1862 at the Battle of Williamsburg. Walker also served at Seven Pines as well as at the Seven Days Battles of the Peninsula Campaign in the summer of 1862 under Maj. Gen. George B. McClellan in the Army of the Potomac. Second Army Corps Walker remained at the Berkeley Plantation until his promotion on August 11 to major and transferral with General Couch to the II Corps of the Army of the Potomac. Although the II Corps later saw action at the battles of Antietam and Fredericksburg, the latter being under the new command of Maj. Gen. Ambrose Burnside, Walker and the Corps did not join Burnsides's Mud March over the winter. Walker was promoted to lieutenant colonel on January 1, 1863, and remained with the II Corps. He fought the Battle of Chancellorsville in May 1863, where his hand and wrist were shattered and neck lacerated by an exploding shell. A record of the 1880 Census indicated that he had "compound fracture of the metacarpal bones of the left hand resulting in permanent extension of his hand". Later in 1896, as