The 71st was assigned to the II Corps in time to participate in the 1862 Peninsula Campaign. During the Seven Days Battles, it made several charges, losing many officers and men. After reaching Harrison's Landing, companies L, M, N, P, and R were merged into the first ten companies. The regiment, along with rest of the Army of the Potomac, was transferred to northern Virginia. Here the 71st fought at the battles of Second Bull Run and Chantilly. It helped cover Pope's retreat. At the Battle of Antietam that September, the regiment lost one third of its strength. It again suffered heavy losses at the Battle of Fredericksburg in December.