Antietam

Yesterday, September 17, was the 146th anniversary of the bloodiest single day in the history of the United States.

For on September 17, 1862, Union and Confederate forces met just to the east of Sharpsburg, Maryland, along a creek known as Antietam.

What might have happened had Lee forced McClellan to retreat from that ground on that day? Assume for example that the union retreat had been managed in good order, and that the Army of the Potomac kept itself thereafter between Lee and the district of Columbia. Assume then that although Lee won the battle, his hypothetical win was itself short of a "knockout."

Still, a victory for Lee's army that day might have been of enormous importance in persuading the UK and France to enter to war on the side of the rebs. It might have been, thus, through diplomatic indirection, a knock-out after all.

Here's a link to a map of the clash.