Who apposed the Battle of Antietam?

Answer 1

It wasn't that kind of battle. Nobody planned Antietam, or advised against it

The opposing sides were the Union troops under McLellan and the Confederates under Lee, who was trying to invade Pennsylvania.

By pure chance, a set of Lee's orders fell into McLellan's hands, and he was well-placed to destroy the Army of Northern Virginia. But there was a Confederate spy in the Union camp, who managed to alert Lee to concentrate his forces.

The battle was an important Northern victory, but very costly, and the Confederates were able to escape back to Virginia.