The Battle of Antietam was the bloodiest one-day battle of the Civil War, with over 25,000 casualties total. It was a Union victory, which raised the confidence of Union troops. After a long winning streak, the Confederate troops lost and were forced back.
One of the main reasons for the civil war was the differences with slaves about whether it was illegal or legal to have slaves. This could have been avoided if there was agreement on how slaves should be treated.
