Antietam

On September 17, 1862, the Battle of Antietam, the deadliesr one day in the Civil War with 22,000 dead, wounded, or missing took place. An excerpt from the article:

"Antietam, the deadliest one-day battle in American military history, showed that the Union could stand against the Confederate army in the Eastern theater. It also gave President Abraham Lincoln the confidence to issue the preliminary Emancipation Proclamation at a moment of strength rather than desperation.

How it ended
Inconclusive. General Robert E. Lee committed his entire force to the battle, while Maj. Gen. George B. McClellan sent in less than three quarters of his. With the full commitment of McClellan’s troops, which outnumbered the Confederates two to one, the battle might have had a more definitive outcome. Instead, McClellan’s half-hearted approach allowed Lee to hold ground by shifting forces from threat to threat."