The Great War
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Source: britannica.com
8-9% of their population died in the war, the third largest proportion of any belligerent (after the Ottomans and Serbia at 13-15% and 17-28%, respectively.)
In an interesting contrast you can see how different the situation was in the later part of the war – the US’s casualty rate is incredibly low (8%). By 1918 when the bulk of the American troops were arriving, there were no more stupid over the top mass charges wasting lives. Tactics had evolved greatly and the Hundred Days Offensive was basically just Germany folding totally and going on a full retreat after a couple of Allied successes, giving very little chance for Americans to even suffer casualties.
World War I deaths as a percentage of each nation’s population