Another of McClellan's failings at Antietam (and other battles), was that his intelligence network was bad - run by Allen Pinkerton (later the founder of the famous Pinkerton Detectives). Pinkerton and his people were terrible spies and the numbers on Lee's strength where always over inflated. Then when McClellan got the numbers, he inflated them even more. McClellan had just under 90000 men at Antietam. Lee had only about 35000 men, but McClellan was convinced Lee's army numbered over 100000. McClellan was being "courageous" by taking the attack to Bobby Lee even though he was "outnumbered".