After emerging from winter quarters in March of 1864, Harris and his fellow 148th Pennsylvanians were attached to the Fourth Brigade. On April 22, they marched in the Grand Review overseen by Union General Ulysses S. Grant. Marching for the Wilderness on May 4, they next engaged the enemy in the Battle of Po River on May 10 near Spottsylvania. Two days later, on May 12, 1864, Harris performed the act of valor for which he would later receive the U.S. Medal of Honor. While engaged with his regiment in charging the "Mule Shoe", a salient exposed during the Battle of Spottsylvania, he wrested the enemy's flag from a Confederate States Army color-bearer, and then shot a second CSA officer who attempted to retrieve it.