In early April 1862, the Army of the Potomac moved up the Virginia Peninsula in two columns. The Third Corps marched from Fort Monroe toward the Yorktown defenses, and the Fourth Corps advanced toward the Halfway House between Yorktown and Williamsburg. Gen. George McClellan planned to trap the Confederates in a two-pronged maneuver with the Third Corps holding Gen. John Magruder’s forces at Yorktown while the Fourth Corps blocked their retreat toward Williamsburg. The Battle of Lee’s Mill on April 5, 1862, stopped the Union flanking movement. Gen. McClellan’s maps incorrectly showed the Warwick River flowing parallel to the James River, he was unaware that the Warwick blocked the Union advance near Yorktown. Lee’s Mill was defended by one brigade from Brig. Gen. Lafayette McLaws’ division and supported by the Peninsula Artillery. Brig. Gen. William F. Smith’s division led the Union advance from the Warwick Court House toward the mill. Halted by the Confederate defenses, Gen. Smith deployed his three brigades under heavy fire along the Warwick. Brig. Gen. Erasmus Keyes, Fourth Corps commander, reported to McClellan that “No part of this line as far as discovered can be taken by assault without an enormous waste of life.” The Battle of Lee’s Mill only cost a handful of casualties, but it thwarted McClellan’s plans and convinced him to besiege the Warwick-Yorktown line.