What mistake did McDowell make in his attack?

During the battle, McClellan kept four divisions of soldiers in reserve and refused to use them to attack Lee's devastated army. McClellan was convinced that Lee was massing reserves for a counterattack.

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During the battle, McClellan kept four divisions of soldiers in reserve and refused to use them to attack Lee's devastated army. McClellan was convinced that Lee was massing reserves for a counterattack.