American Civil War re-enactors on the summit of Little Round Top looking out over what became, after the Battle of Gettysburg, as "The Valley of Death".
Late on the afternoon of July 3, 1863, going by the numbers I first read, after several hours of a Confederate artillery barrage which overshot the Union lines and was almost completely ineffective, approximately 15,000 Southern men under the command of Gen. George Pickett, started across the mile wide valley into devastating artillery and small arms fire from the Northerners, who were firing from behind a low stone wall.
The few confederates who did get to the union lines were beaten back in horrific hand-to-hand fighting, but only 3,000 made it back to Seminary Ridge.
To put this in perspective: the South left 12,000 casualties on the field on 90 minutes. In the Normandy landings on D-Day, the allies landed 175,000 men in 12 hours and incurred 10,000 casualties.
This, more than anything else I know of, shows the horrors of this war.