American Battlefield Trust's map of the Battle of Gaines’ Mill
In this, the third of the Seven Days’ Battles, Gen. Robert E. Lee renewed his attacks against Fitz John Porter’s Union Fifth Corps. In the early hours of June 27, 1862, Porter’s troops abandoned their position at Beaver Dam Creek and established a new defensive line behind Boatswain’s Swamp, just north of the Chickahominy River. Lee was determined to drive the Army of the Potomac across the river and sent the bulk of his force in search of the Yankees. Shortly after noon, Confederates drove in Yankee skirmishers and encountered stiff resistance along Boatswain’s Swamp. The Federals beat back successive waves of disjointed Southern troops, inflicting some of the heaviest casualties the war had yet seen. By dusk, however, Lee’s Confederates were more organized. With daylight fading, the reinforced Southerners assaulted Porter’s anemic defensive line and sent the Northerners fleeing toward the river. Only the approaching darkness prevented Porter’s corps from being decimated. During the night, the Federals limped across the Chickahominy and burned the bridges behind them. The defeat at Gaines’ Mill convinced McClellan to abandon the campaign against Richmond and “change his base” to the James River. Gaines’ Mill was the most sanguinary engagement of the Seven Days’ battles.