Final answer:
The Battle of Antietam during the American Civil War in 1862 was the deadliest day in American history with over 20,000 soldiers killed, wounded, or missing in just twelve hours.
Step-by-step explanation:
The subject of this question is History. The question is asking about the deadliest day of the Battle of Antietam, which occurred during the American Civil War in 1862. The battle took place in Maryland, and it remains the bloodiest single day in American history, with over 20,000 soldiers killed, wounded, or missing in just twelve hours.