Burnsides forces moved up the hill and began pushing the Confederate forces back into the town of Sharpsburg. Fighting was fierce, but superior Union numbers gave them the advantage until Maj. Gen. A.P. Hill arrived on the scene from Harper's Ferry. (He marched his light artillery division 17 miles in 8 hours.) Union forces retreated to a defendable position above town.

Both General McClellan, commander of the Army of the Potomac and General Burnside incorrectly believed they were outnumbered by Lee's Army of Northern Virginia, so neither would press an attack the following day. Lee withdrew his forces to Virginia overnight Sept. 18/19.