On September 15th, 1862 Jackson won at Harper's Ferry, but two days later came the battle of Antietam, when Confederate forces under General Lee were caught by McClellan near Sharpsburg, Maryland. This was the bloodiest day of the war.
I have been there and seen what they call "Bloody Lane" where the brave Confederates fell by the thousands. I have climbed the observation tower. It is a sobering sight to think of what went on there. 2108 Union soldiers died and 9549 were wounded. And 2700 Confederates were killed and 9029 wounded. The battle had no clear winner but since Lee had to withdraw to Virginia, McClellan was considered the victor.
This caused the British and French who were contemplating official recognition of the Confederacy to reserve action, and gave Lincoln to announce his Preliminary Emancipation Proclamation on Sept 22, so this battle was more decisive in long-term meaning than it would have first appeared.