What is the Antietam battle about?

Answer 1

It was Lee's first attempt to invade the North, and threaten Washington DC.

The Union, under McLellan, gained an advantage by procuring a set of Lee's orders, with details of the movements of the separate parts of his army. They thought they would be able to destroy each part in turn. But there was a Confederate spy in the camp, who alerted Lee, and the Confederates were able to re-group.

The North won the ensuing battle at Antietam Ceek, but was not able to destroy Lee's army, which escaped back to Virginia.

The significance of the battle is that it represented the failure of Lee's campaign, which was being taken as the test of Confederate viability by the British parliament. So the British decided not to intervene on the side of the South.