As we commemorate the 157th anniversary of the Battle of Antietam on September 17, we shouldn’t forget that on both the 16th and the 18th a Union attack of any size would have decimated Lee’s Army of Northern Virginia. On the 16th almost half of his forces were not in place in Sharpsburg, but McClellan let the fog and indecisiveness stop him from attacking. On the 18th Lee’s forces were bloodied, unbowed, but ready to be overwhelmed because of the massive casualties of the previous days. McClellan, with major reinforcements on the way and many troops still fresh (not active the day before), instead did nothing and let Lee escape.
Had McClellan attacked in force either day, Antietam could have been THE turning point of the war as Lee and his men would have been overwhelmed. Instead we had two more years of death and destruction.
Yes, “what if” is always an interesting discussion. In fact, that’s why I wrote my second book about just such a scenario. But it describes a Union victory on the 17th. Victory on the 16th and 18th, in k20-20 hindsight, admittedly, were there for the asking.