The Germans (
just the Germans) lost two million dead, and many more wounded. If you add the other Axis powers, then it looks even worse.
Consider, also, that there were numerous fronts in the war. The Russians lost almost as many as the Germans, but they were on a totally different front from the European allies, so maybe we could say the Russian trenches were the worst, but I'll bet that they had a lot of Cossack cavalry; which, I suspect, didn't do so well against machine guns.
The Ottomans had a huge civilian death toll, but that included the "Armenian Issue," which people like to fight about (My wife actually had a neighbor that was a survivor of that "Issue," and he called it "genocide").
The short of it, was that no single nation suffered as many casualties as Germany. It's notable that they suffered so few direct civilian casualties (but look what happened from secondary causes). Boy, did that change in WWII.
The only nation that came close to Germany for total deaths was Russia.
Then, of course, we had the good ol' 1918 Pandemic, and everybody took that in the shorts.