The correct answer is "It suffered by far the most casualties".
After the end of World War, I in 1919, the country which had the most human losses regarding soldiers was France. Out of the 8 million men that France prepared for prior to the war, approximately 1.4 million died. This represented a slump of 3.5% of the total population of the country. This had a massive impact on French society, as nearly everyone was related to families which had suffered the loss of their relatives as a consequence of the war.
