The Battle of Antietam

Even though the confederate strategy was to defend, General Robert E Lee advanced his troops into Maryland. At this point and time, General McClellan and his troops had two major advantages over the confederates. Firstly, the union forces were double the confederate forces, and secondly McClellan had a copy of Gen. Lee's battle plans. 

The first day of the Battle of Antietam was the bloodiest day of the civil war with 22,000 total casualties.

This victory for the Union was incredibly important and aided them greatly in the war. The union had not only repelled the south from invading the north, but also the European powers that were considering helping the Confederate shied away.

One other result of the Battle of Antietam was that it gave Lincoln the courage to issue the Emancipation Proclamation.

The Battle of Antietam is connected to the Battle of Gettysburg, as the latter was the South's second attempt to invade the North.