 The battle was tactically inconclusive , although Lee technically was defeated because he withdrew first from the battlefield and retreated back to Virginia . McClellan wired to Washington , " Our victory was complete . The enemy is driven back into Virginia . " Yet there was obvious disappointment that McClellan had not crushed Lee , who was fighting with a smaller army with its back to the Potomac River . Although McClellan 's subordinates can claim their share of responsibility for delays ( such as Ambrose Burnside 's misadventures at Burnside Bridge ) and blunders ( Edwin V. Sumner 's attack without reconnaissance ) , these were localized problems from which the full army could have recovered . As with the decisive battles in the Seven Days , McClellan 's headquarters were too far to the rear to allow his personal control over the battle . He made no use of his cavalry forces for reconnaissance . He did not share his overall battle plans with his corps commanders , which prevented them from using initiative outside of their sectors . And he was far too willing to accept cautious advice about saving his reserves , such as when a significant breakthrough in the center of the Confederate line could have been exploited , but Fitz John Porter is said to have told McClellan , " Remember , General , I command the last reserve of the last Army of the Republic . " 
