Who lost the most lives

Is this a homework question?

There are many books on WWI. Just go to your local library and ask the librarian where you can find books on that subject. You can also Google, there's information on everything and anything on the internet. It just takes a bit of time reading through the info.

You can handle this research on your own. If this is homework we won't do it for you, it's one of the rules of this site. If you're just curious, the information is out there with just a few clicks of the keyboard. :)

Good luck.