the remainder of his army under Maj. Gens. A.P. Hill, James Longstreet, and D.H. Hill attacked from the direction of Mechanicsville. The Seven Days Battles began with a Union attack in the minor Battle of Oak Grove on June 25, but the first major battle started the next day when Lee launched a large-scale assault against McClellan at the Battle of Beaver Dam Creek (or Mechanicsville). Lee attacked Porter's V Corps north of the Chickahominy, while the bulk of the Union Army was relatively unoccupied south of the river. Although this battle was a tactical victory for the Union, McClellan realized that he could not keep Porter's corps in place with Jackson threatening his flank. He ordered Porter to begin a withdrawal and at the same time decided to change the army's base of supply from White House on the Pamunkey River to Harrison's Landing on the James River. (This decision was fatal to McClellan's campaign because by abandoning the railroad that led from the Pamunkey, he would no longer be able to supply his planned siege of Richmond with the necessary heavy artillery.) Several of McClellan's subordinates urged him to attack the Confederate division of Maj. Gen. John B. Magruder south of the Chickahominy, but he feared the vast numbers of Confederates he believed to be before him and failed to capitalize on the overwhelming superiority he actually held on that front. Magruder assisted in this misapprehension by ordering frequent, noisy movements of small units back and forth and by using groups of slaves with drums to simulate large marching columns. Furthermore, the Union Army Balloon Corps, which had performed the only aerial observation during the Peninsula Campaign, was now joined by a Confederate competitor. Capt. Langdon Cheves of South Carolina had constructed a multicolored balloon of dress silk obtained from Charleston and Savannah, which sailed aloft tethered to a boxcar on the York River Railroad, manned by Maj. Edward Porter Alexander. The appearance of this balloon reinforced McClellan's fear that the Confederates were planning for an offensive against his left flank. For the second day, the Confederates were able to continue fooling McClellan south of the river by employing minor diversionary attacks to command the attention of 60,000 Federal troops while the heavier action occurred north of the river. The order to Porter's corps came just before dawn and they did not have adequate time to prepare a