Once a politician like Trump has been successful in creating a tribal mentality, he can more easily manipulate his followers and trick them into viewing facts as "fake news" given that he has successfully created an us vs them mentality. He can then use propaganda to brainwash his followers given that they will tend to view true facts as "fake news." This sort of tribal mentality leads to people more easily manipulated and brainwashed and thus leads to hate and of course, hate leads to evil acts such as war and genocide.
So, wars and genocide have as their origins politicians like Trump for example who exploit the human nature of tribalism to divide people up so they can be more easily manipulated and brainwashed so that they can gain and obtain power at the expense of their followers as well as others who might not be in their tribe. Human nature is tribal and political manipulators exploit this to gain power even at the cost of others.