That's not really fair, comparing the total manpower of battles with a time difference of about 600 years isn't it? For that time it would probably be nearly impossible to maintain such a large army. You'll have to give credit to the Mongolians that from a couple of fighting tribes they managed to put up a force that would create one of the largest empires that the world has ever known.
And I'm pretty sure that more people were involved in the Mongolian conquest, the battles were fought in an area from Poland to Japan on two different continents for several decades and even influenced language (in Dutch, 'Mongool' - Mongolian is also being used as a swearing word). On top of that I doubt that during the ACW any commands were given to completely wipe out cities, or even civilizations.
I do not really see the extreme significance of the ACW, besides it had a major influence on the history of the USA. Of course, it must have had quite an impact on military development but that counts for just about every major conflict that has taken place. Just think about for example the V2 rockets and the jets that Germany developed during WWII or the simple but brilliant tank design of the Soviets.
I think the only reason I've received lessons about the ACW at high school is because it was explained that it was about getting rid of slavery - which appears to be false, or at least had a way more minor role in it than I was teached. Or at least, some American told me that. But please, correct me if I'm wrong on that subject. I do not know that much about the ACW.
I'm not sure about the modern part thing. Indeed some new weaponry and tactics were used, but WWI introduced some technology that would determine all of the following wars: planes, tanks, chemical weapons. Shiny outfits also disappeared. But I think that is really a matter of opinion as the ACW also had plenty of unique new things. If I'm correct the ACW saw the first submarine (with an unpleasant journey for the crew), right?
I don't want to be a smartass, but also about the photography during the ACW - the camera's at that time weren't developed enough yet to capture fast movement. It was first used during that war - but as far as I understand it this wasn't on a large scale yet or at least not useful to photograph the battles. Though it was one of the last wars in which drawers were used on a large scale (I've read this in the National Geographic of May - the Netherlands).