Meanwhile, Chamberlain makes his transition from teacher to military officer and practices drilling his soldiers, and is taught military tactics by Col. Adelbert Ames, the commander of the 20th Maine Volunteer Infantry Regiment. He is called to battle at the Union invasion of Fredericksburg. The Southern forces lead a fighting retreat as the Union army crosses the river to storm and loot Fredericksburg. Outside the city, Lee, James Longstreet and Jackson have prepared an elaborate defense on Marye's Heights outside the town, and the movie focuses on Confederate defenses behind a formidable stone wall. Several Union brigades, including the Irish Brigade, attempt to cross an open field and attack the wall, but are thrown back with heavy losses by Confederate rifle and artillery fire. At one point, two Irish units are forced into battle against one another, to the anguish of a Southern Irishman who believes he is killing his kin. Chamberlain leads an unsuccessful attack against Longstreet's defenses, led by Brig. Gen. Lewis Armistead and finds his unit pinned down in the open field. He survives by shielding himself with a corpse until nightfall; eventually he and surviving members of 20th Maine are ordered to retreat and spend 2 nights on the battlefield, sleeping with the dead. Chamberlain and the defeated Union soldiers depart Fredericksburg. Jackson and Lee return to the city, and Lee is confronted by an angry citizen whose house has been destroyed by Union artillery.