You are aware that casualties are different from deaths?
Total dead at Antietam was about 3500. I have no idea if this is the most in a single day for American troops, but it’s not a whole lot more than died on 9/11. Pearl Harbor was 2500.
Most troops dead during the war died from disease, 2x to 3x those who died as a result of combat. Most troops were farmboys. Bring them all together in unsanitary conditions and they all swapped diseases and a whole bunch died. Many regiments lost over 50% before they saw combat.
Which means we consistently under-estimate the violence of recent US wars. To a very considerable extent the lower death rate recently measures not the intensity of combat but better medical care.
Same is true of the murder rate. Recent declines are at least as much due to better emergency care as to less violence.