The Battle of Antietam—an 1862 clash between Robert E. lee's Army of Northern Virginian and George McClellan's Army of the Potomac—was the deadliest one-day battle in American history, with a total of 22,717 dead, wounded or missing. It came after Lee thwarted McClellan's plans to lay siege to the Confederate capitol of Richmond and tried to seize the momentum by crossing north into Maryland.