One of the key advantages for Union forces at Antietam was their superior numbers.

The Army of the Potomac, under General George McClellan, outnumbered the Confederate Army of Northern Virginia, commanded by General Robert E. Lee. This numerical advantage allowed the Union to launch multiple assaults and maintain a sustained presence on the battlefield, despite facing fierce resistance. Additionally, having more troops enabled the Union to absorb casualties more effectively and to rotate fresh units into the fight. This strength in numbers played a crucial role in the intense and bloody engagement, contributing to the Union's ability to achieve a strategic victory.