What discovery led to george b. mcclellan's victory over robert e. lee at antietam?

The correct answer to this open question is the following.

The discovery that led to George B. McClellan's victory over Robert E.Lee at Antietam was that McClellan found General Robert E. Lee's battle plan and that factor gave him a clear advantage.

During the actions of the American Civil War, the Battle of Antietam represented a major loss to the Confederate Army led by General Robert E. Lee. The Southern Army was trying to invade the North. The battle started on September 17, 1862, in Antietam Creek, Maryland. Although it only lasted one day, this was a bloody battle in which approximately 3,600 soldiers died.

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