Re-engaging with the enemy the next day (August 30), the 48th Pennsylvanians experienced more intense combat during the Battle of Chantilly (September 1), during which the 48th Pennsylvania was again positioned at the far right of the Union Army. Ordered to Alexandria after the battle's conclusion, the regiment was reassigned to the Maryland Campaign (September 4–20). After fighting again near Fox's Pass in the Battle of South Mountain (September 14), Monaghan and the 48th Pennsylvania persevered through the brutality of the Battle of Antietam (September 17). Having engaged the enemy that morning at Burnside's Bridge, they then led the 1st Brigade in its pursuit of CSA troops through Sharpsburg and its surrounding bluffs while under heavy enemy artillery and rifle fire. Continuing to march and skirmish periodically with the enemy that fall, they then fought again in the Battle of Fredericksburg (December 11–15).