What was the main idea of the Battle of Antietam?

Answer 1

There was no plan for this battle.

Following a string of victories in Virginia, Lee was planning to invade Pennsylvania.

He had split his army into several parts, and managed to keep the enemy in the dark about his position.

By pure chance, a copy of his orders had been dropped in the field, and they were found by Union troops and shown to their commander, General McLellan, who prepared to destroy each of the divisions, one by one.

But he did not move quite fast enough, and Lee had time to concentrate his troops at the nearest place, which was Antietam Creek, Sharpsburg.

It resulted in a Confederate defeat (Lee's first), with important implications for the outcome of the war, although McLellan was criticized for failing to pursue Lee after the battle.