For much of its first two years, the 70th Indiana performed reconnaissance duty and guarded railroads in Kentucky and Tennessee. In 1864, Harrison and his regiment joined William T. Sherman's Atlanta Campaign and moved to the front lines. On January 2, 1864, Harrison was promoted to command the 1st Brigade of the 1st Division of the XX Corps. He commanded the brigade at the battles of Resaca, Cassville, New Hope Church, Lost Mountain, Kennesaw Mountain, Marietta, Peachtree Creek, and Atlanta. When Sherman's main force began its March to the Sea, Harrison's brigade was transferred to the District of Etowah and participated in the Battle of Nashville.