Jonah, this is an old controversy, but while your reader is right (America did put an end to the Great War), it can also be argued that the hope of America’s involvement prolonged that war by a year or two. Churchill, amongst others, said later that it might have been better if America had not entered the war. A fair peace negotiated between the exhausted protagonists in, say, 1916, might have left Europe (most notably, Russia) in far better shape than the victory of 1918. I’m not entirely convinced, but it’s an interesting thought…