 The Battle of Antietam on September 17 , 1862 , was the single bloodiest day in American military history . The outnumbered Confederate forces fought desperately and well . Despite significant advantages in manpower , McClellan was unable to concentrate his forces effectively , which meant that Lee was able to shift his defenders to parry each of three Union thrusts , launched separately and sequentially against the Confederate left , center , and finally the right . McClellan was also unwilling to employ his ample reserve forces to capitalize on localized successes . Historian James M. McPherson has pointed out that the two corps McClellan kept in reserve were in fact larger than Lee 's entire force . The reason for McClellan 's reluctance was that , as in previous battles , he was convinced he was outnumbered . 
