Apparently the authors never heard of the civil war.
I was just getting ready to mention that. Civil war battles would have dominated this list had they been included in the count.
It should be no surprise that the deadliest battles fought on American soil came about during the American Civil War, where the battlefields of Gettysburg, Shiloh, Stones River and so many others yielded staggering casualty lists.
(But what fun would FR be if we started reading articles before commenting on them?)