Outnumbered two-to-one, Robert. E. Lee committed his entire force. McClellan sent in less than three-quarters of his army, enabling Lee to fight the Federals to a standstill.
McClellan's attacks failed to achieve force concentration, allowing Lee to counter by shifting forces and moving interior lines to meet each challenge.
The photo on the right shows Alan Pinkerton, Union Intelligence Service, Abraham Lincoln, President, and George B. McClellan, commander of union forces just after the battle.
The battle was over by 5:30 p.m. ending twelve hours of savage combat. The Union had 12,401 casualties with 2,108 dead. Confederate casualties were 10,318 with 1,546 dead. This represented 25% of the Federal force and 31% of the Confederates on the field that day.
More Americans died on September 17, 1862, than on any other day in the nation's military history. Hence the nickname 'Bloody Antietam'.
Consider that this level of slaughter was created using weapons primitive by modern warfare standards - Model 1861 Percussion Rifle-Musket (A single-shot, muzzle-loading gun detonated with a percussion cap, .58 caliber bullet (minie ball), effective range 300 yards).
Today, the battlefield is hallowed ground, managed by the US national park service.