The military innovation noted at Antietam was the use of repeating rifles.

These firearms allowed soldiers to fire multiple rounds without reloading after each shot, significantly increasing their firepower compared to traditional muzzle-loading rifles. This technological advancement had a profound impact on the battlefield, contributing to the high casualty rates during the battle on September 17, 1862. The increased accuracy and rate of fire from repeating rifles changed the dynamics of infantry combat, making engagements more deadly and leading to the need for new tactics to adapt to this evolving warfare landscape in the Civil War.