At Antietam: General Lee was successful and pushed north into Pennsylvania. General McClellan surrendered his troops. the nation suffered more casualties than on any other day in its history. the Union's river fleet proved crucial to the outcome. Lincoln announced the Thirteenth Amendment.

At Antietam the nation suffered more casualties than on any other day in its history.

Explanation:

The Battle of Antietam, also known as the Battle of Sharpsburg, was an armed clash that took place on September 17, 1862 near the city of Sharpsburg, Maryland in the United States during the Civil War.

Total losses amounted to over 23 thousand soldiers in one day of battle. It was the bloodiest day in all of US history.

This battle is considered a breakthrough moment in the war, because from then on, despite a number of further victories, the advantage of the Southerners began to decrease markedly, which eventually led to the Union's victory two and a half years later.