The Union Army straddled the rain-swollen Chickahominy River. Four of the Army's five corps were arrayed in a semicircular line south of the river. The V Corps under Brig. Gen. Porter was north of the river near Mechanicsville in an L-shaped line running north–south behind Beaver Dam Creek and southeast along the Chickahominy. Lee moved most of his army north of the Chickahominy to attack the Union north flank. This concentrated about 65,000 troops against 30,000, leaving only 25,000 to protect Richmond against the other 60,000 men of the Union army. It was a risky plan that required careful execution, but Lee knew that he could not win in a battle of attrition or siege against the Union army. The Confederate cavalry under Brig. Gen. J.E.B. Stuart had reconnoitered Porter's right flank as part of a daring circumnavigation of the entire Union army from June 12 to June 15 and found it vulnerable. Stuart's forces burned a couple of Union supply ships and was able to report much of McClellan's army's strength and position to Gen. Lee. McClellan was aware of Jackson's arrival and presence at Ashland Station, but did nothing to reinforce Porter's vulnerable corps north of the river.

Lee's plan called for Jackson to begin the attack on Porter's north flank early on June 26. Maj. Gen. A.P. Hill's Light Division was to advance from Meadow Bridge when he heard Jackson's guns, clear the Union pickets from Mechanicsville, and then move to Beaver Dam Creek. The divisions of Maj. Gens. D.H. Hill and James Longstreet were to pass through Mechanicsville, D.H. Hill to support Jackson and Longstreet to support A.P. Hill. Lee expected Jackson's flanking movement to force Porter to abandon his line behind the creek, and so A. P. Hill and Longstreet would not have to attack Union entrenchments. South of the Chickahominy, Magruder and Huger were to demonstrate, deceiving the four Union corps on their front.

The Battle of Beaver Dam Creek, also known as the Battle of Mechanicsville, took place on June 26, 1862, in Hanover County, Virginia, was the first major engagement of the Seven Days Battles during the Peninsula Campaign of the American Civil War. It was the start of Confederate General Robert E. Lee's counter-offensive against the Union Army of the Potomac, under Maj. Gen. George B. McClellan, which threatened the Confederate capital of Richmond. Lee attempted to turn the Union right flank, north of the Chickahominy River, with troops under Maj. Gen. Thomas J. "Stonewall" Jackson, but Jackson failed to arrive on time. Instead, Maj. Gen. A.P. Hill threw his division, reinforced by one of Maj. Gen. D.H. Hill's brigades, into a series of futile assaults against Brig. Gen. Fitz John Porter's V Corps, which occupied defensive works behind Beaver Dam Creek. Confederate attacks were driven back with heavy casualties. Porter withdrew his corps safely to Gaines Mill, with the exception of Company F (a.k.a. The Hopewell Rifles) of the 8th Pennsylvania Reserve Regiment who did not receive the orders to retreat.