Final answer:
The Battle of Antietam, occurring on September 17, 1862, is known as the bloodiest single day battle in American history with over 20,000 combined casualties. In comparison, the Battle of Gettysburg had the most overall casualties in the Civil War but spanned three days.
Step-by-step explanation:
The bloodiest single day battle in American history took place during the Civil War and is known as the Battle of Antietam. This monumental conflict occurred on September 17, 1862, near Sharpsburg, Maryland, alongside Antietam Creek. Although the Union forces under Major General George B. McClellan and the Confederate army led by General Robert E. Lee sustained heavy losses, the battle was particularly significant as it marked the single day with the highest number of casualties in US military history. Over 20,000 soldiers were killed, wounded, or went missing within just a twelve-hour period. This battle was a strategic moment in the war as it led to President Abraham Lincoln issuing the Emancipation Proclamation, which declared all slaves in Confederate-held territory to be free.
Comparatively, the battle with the most overall casualties was the Battle of Gettysburg, which lasted three days and resulted in combined casualties nearing 51,000. While Antietam holds the record for a single day, Gettysburg remains the most substantial battle in terms of total casualties during the entire Civil War. Despite Antietam being a tactical draw, the Union's strategic advantage eventually helped pave the way for victory in the war.