World War II was complicated, especially on the eastern front. The Ukrainians were trapped between Hitler and Stalin, both of whom wanted them exterminated, and nobody had any way of knowing which one history would be written by. Frankly, I find it completely par for the course to get tangled up trying to figure out who was the good guys, I don't see how this particular gaffe is such a big deal.

Perhaps you can add it to a pile of other existing gaffes and call that the final straw, but without that pile already existing this wouldn't really register.