Yes, technically it is true that the Byzantine emperor, who believed that Muslims posed a threat to Christian Constantinople, requested help from Europe, although he mostly dealt with them himself.
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By the numbers, the WWII was indeed the most deadly war in history as it killed an estimate of 60 to 75 million people. It was the first war that really took multi-continental proportions and affected every country and group of people in the world. It involved around 100 million people and it was marked by mass killings of civilians, like the Holocaust, and atomic bombings. The Soviet Union was the country that suffered the most casualities, civilian and military, losing roughly 15% of its population.
