On September 14, at the base of South Mountain near Sharpsburg, Confederate Generals D.H. Hill’s and James Longstreet’s units encountered Union resistance and sustained heavy casualties. Lee planned to retreat to Virginia, but changed his mind after hearing Confederate General Thomas Jonathan Jackson had captured Harper’s Ferry. Instead, Lee ordered his army to regroup at Antietam Creek near Sharpsburg.
Troops from both sides faced-off across a 30-acre cornfield owned by David Miller. Union troops fired first at the Confederate’s left flank and the carnage began. Confederate troops ferociously fought off offensive after offensive to prevent being overrun, turning the cornfield into a massive killing field.
The next day, as Lee began the painstaking job of moving his ravaged troops back to Virginia, McClellan, surprisingly, did nothing. Despite having the advantage, he allowed Lee to retreat without resistance.