re: Is what the US did in WWII the most impressive thing a country’s ever done?

Not saying they won the war singlehandedly (far from it), but the fact is that roughly 80% of the Nazi war machine's resources were tied up on the Eastern Front. It would have been MUCH tougher to land in France had the bulk of the Wehrmacht not been occupied by the Soviets on the Ost Front. Just being fair and honest. The USSR suffered between 20 and 30 million war deaths fighting the Nazis.