At the Battle of Monocacy (July 9, 1864), Early defeated a smaller force under Lew Wallace near Frederick, Maryland, but this battle delayed his progress enough to allow time for reinforcing the defenses of Washington. Early attacked a fort on the northwest defensive perimeter of Washington (Fort Stevens, July 11–12) without success and withdrew back to Virginia. He successfully fought a series of minor battles in the Valley through early August and prevented Wright's corps from returning to Grant at Petersburg. He also burned the city of Chambersburg, Pennsylvania, retaliating against Hunter's earlier actions in the Valley.