JTL317 is probably correct as The Battle of Somme (WW1) had 1.5 Million casualties over 6 months,19,240 on the first day. Close to a half a million died in battle of Verdun during WW1 but that was over 11 months. The Battle of Stalingrad had 478,741 men killed over a 6 month period. But on American soil it was here in America at the Battle of Gettysburg. At Gettysburg on July 1, 2, and 3, 1863, more men actually fought and more men died than in any other battle before or since on North American soil. Over 172,000 men and 634 cannon had been positioned in an area encompassing 25 square miles. Additionally, an estimated 569 tons of ammunition was expended and, when the battle had ended, 5,000 dead horses and the other wreckage of war presented a scene of terrible devastation.
You may be hard pressed to find a more devastating battle anywhere else at any other time. That was in 3 days, on July 4th,1863 Lee retreated. Leaving 51,000 human casualties. You may be hard pressed to find a more devastating battle anywhere else at any other time. Wars - yes, battles - no. I think you have to look at the time involved as to devastating battles.