How did union leadership completely spoil an opportunity to win the war on september 17, 1862?

Major General George B. McClellan was able to obtain full details of Lee's plans but he failed to act until it was too late. By the time McClellan took action 18 hours later, Lee had already placed his troops in strategic positions around Antietam Creek. McClellan's troops outnumbered the Confederate forces by three to one but he was over cautious and failed to exploit this advantage. 

Answership building; new england was almost entirely mercantilist