According to Lee’s plan, Jackson was to march from Ashland on June 25 and encamp that night just west of the Central Railroad. At 3 a.m. on the 26th he was to advance and envelop Porter’s right flank at Beaver Dam Creek. Then, wrote Lee, “A. P. Hill was to cross the Chickahominy at Meadow Bridge when Jackson’s advance beyond that point should be known and move directly upon Mechanicsville.”

But from the beginning, unforeseen circumstances upset the operation and timing of this plan. McClellan suspected Jackson’s approach, so the element of surprise was lacking. And when the action of the Union pickets in destroying bridges and felling trees in Jackson’s path, as well as the fatigue of his weary troops, combined to delay him, the all-important time element was lost.

As the day wore on with no word from Jackson, A. P. Hill became impatient and fearful for the success of Lee’s plan. He decided to attack regardless. At 3 that afternoon he crossed the Chickahominy and swept the Union outposts from Mechanicsville, thus clearing the way for D. H. Hill and Longstreet’s troops to cross. Porter withdrew to a prepared position behind Beaver Dam Creek, a mile east of Mechanicsville. This naturally strong position was further fortified by felled trees and the banks of a millrace. Here, atop the high banks of the stream, he placed Gen. George McCall’s division, extending from near the Chickahominy on the south, across Old Church road (now U.S. 360) on the north. Gen. Truman Seymour’s brigade held the left and Gen. John Reynold’s the right, with Gen. George G. Meade’s brigade in reserve. The only approaches to the position were across open fields, commanded by the Federal artillery, and down the steep banks of the stream, covered by the soldiers’ muskets.