Yes for the US it is 600,000 deaths(still more than all the us wars combined) that time it was quite a shock especially to the public the staggering amounts of casualties, like some battles Antietam 6000 dead in one day and the losses not only to battle but diseases too. When I did my project on the civil war I found that about little more half the deaths were attributed to disease throughout the war, by the end I think the losses were 2% of the population at that time. But with respect to many of the wars that have taken place I don't think the US civil war would come be at the top of the list. There are of course the world wars and there are a few Chinese rebellions where 20+ million died.