Enlisting for Civil War military service during the summer of 1862, Buck enrolled on August 17 at Windsor, Connecticut, and officially mustered in as a private with Company A of the 21st Connecticut Volunteer Infantry. Assigned initially to the Union's Army of the Potomac, he and his fellow 21st Connecticut Volunteers fought in the Battle of Fredericksburg from December 12–15 of that year. While engaged in Burnside's "Mud March", Buck was promoted to the rank of corporal on January 20, 1863. He and his regiment then participated in the Siege of Suffolk that spring, provost and guard duty near Portsmouth and Newport News that summer, fall and winter. Part of the Union's expedition along Virginia's James River, which launched in late January 1864, the 21st Connecticut Volunteers then passed through Fort Powhatan and Smithfield before heading for North Carolina, where they were on duty at New Bern (February 12), Plymouth, Washington and Blount's Creek before returning to Portsmouth, Virginia in late April. Attached to the Union forces participating in operations against Petersburg and Richmond for much of the spring and summer, they were involved at Swift Creek, Fort Darling, the Battle of Drewry's Bluff, Bermuda Hundred, and White House before fighting in battles near Cold Harbor for much of June and in the trench warfare at Bermuda Hundred from late August through late September 1864.