The bloodiest day in U.S. military history as Gen. Robert E. Lee and the Confederate Armies are stopped at Antietam in Maryland by McClellan and numerically superior Union forces. By nightfall 26,000 men are dead, wounded, or missing. Lee then withdraws to Virginia.This was in about 12 hours of fighting.
Of course the fact that McClellan was in charge of Union forces meant not all the troops were committed. And after a day of burying the dead and dealing with wounded, Lee's Army of Northern Virginia was allowed to retreat into Virginia, ending Lee's first invasion of the North.