The Battle of Antietam was Robert E. Lee's first invasion of the North. He divided his men to try and divert his attention across the Union states, but that proved to be the wrong decision.
Abraham Lincoln caught wind of Lee's plans to move up through the southern states through Northern Virginia. So, Lincoln appointed Major General George B. McClellan to take charge of the Union troops that were responsible for defending the capital against the invasion. Over the course of September 15 and 16, Confederate and Union forces faced each other from across Antietam Creek, waiting to clash.
The battle began at dawn on a foggy September 17th. The combat was far from "honorable warfare," waging for eight hours across the river. By the end of the day, the Confederates got the worst of the trade. They were shaken up, but not beaten, despite losing about 15,000 soldiers.
Combined, both armies lost roughly 23,000 of the 100,000 that engaged in combat. More than 3,600 total died. On the morning of September 18th, both sides buried their dead and tended to the wounded soldiers. That night, Lee turned right around and led his army back to Virginia.
Since this was a single day battle, the numbers pale in comparison to fights like the Battle of Gettysburg. But, in the few hours that the fighting commenced, 23,000 total casualties within 100,000 participants means that there was a 23% casualty rate. Compared to other bloody battles that sit in at roughly 19% on the high end, this was a huge moment for history.