I've done a considereable amount of reading about the history of America's wars, including the Civil War, and if I remember correctly, there were more Americans killed in that war than in all the others combined, including World Wars I and II. A tragic shame, really.
I guess the old memory is still working okay as the "Introduction" of this report confirms there were more American battle deaths (on both sides) in the U.S. Civil War than in all the others.