In early April, 1862, the Army of the Potomac moved up the Virginia Peninsula from Fort Monroe in two columns. On the right, the III Corps marched toward the Yorktown defenses, and on the left the IV Corps advanced toward Williamsburg. Maj. Gen. George B. McClellan planned to trap the Confederates in the two-pronged maneuver with the III Corps holding Maj. Gen. John Magruder’s forces at Yorktown while the IV Corps under Brig. Gen. Erasmus Keyes blocked their retreat toward Williamsburg. McClellan’s maps, however, incorrectly showed the Warwick River flowing parallel to the James River; he was unaware that the Warwick partially blocked Keyes’ advance. Lee’s Mill on the Warwick 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 toward the mill. Halted by the Confederate defenses on April 5, Smith deployed his three brigades under heavy fire along the Warwick. Keyes 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 cost few casualties, but thwarted McClellan’s maneuver plans and convinced him to besiege the Confederates.