How did WWI undermine the myth of European invincibility?
World War I undermined the myth of European invincibility by showing how the Europeans were not morally or culturally superior. One of the major defenses of European imperialism had been the idea that the Europeans were superior to (in particular) non-white people in these ways. But WWI showed that the Europeans were fully capable of acting very foolishly in ways that would end up practically destroying themselves. The fact that the Europeans ended up in this suicidal war helped to disabuse people of the notion that they were in some way better than other people.