The Battle of Antietam showed the Union that?

                                             The Battle of Antietam

The Battle of Antietam, also called the Battle of Sharpsburg, occurred on September 17, 1862, at Antietam Creek near Sharpsburg, Maryland. It pitted Confederate General Robert E. Lee’s Army of Northern Virginia against Union General George McClellan’s Army of the Potomac and was the culmination of Lee’s attempt to invade the north. The battle’s outcome would be vital to shaping America’s future, and it remains the deadliest one-day battle in all of American military history.

true true true true that all

the answer is prolly b