They're not being tossed in a blender and redispersed throughout their country, they're already settled.
Dividing up territories without logic, placing some groups under the thumb of a larger majority which might be the minority and thus roles reversed in a neighboring country, pitting these people against each other in basic divide and conquer manuevers (a tactic the English in particular overly relied on), ridding them of customary systems of interrelating, favoring some groups over others, utilizing people as proxies in area conflicts or to foment such... these things lay the seeds which can grow fetid over time.
The world's two oldest countries - China and Iran - have been multi-ethnic and multi racial since before they became countries.
Rome was likewise mixed not just by native populations but through their conquests moving about the empire as citizens. Egypt as well. Same goes for Britain, Russia... oh, and the USA. In fact, very few nations don't have some sort of mixture.
History's largest and most powerful nations have not only been mixed, but that very mixture fueled much of the innovation and progess that they became known for and that separated them from their less impressive neighbors.
However, subjugating and exploiting people via oppressive regimes from abroad only to be replaced with heavy handed autocratic rule or govermental structures easily toppled by such, stripping lands of resources later generations must then compete with each other for, as well as residual feelings of resentment towards those treated more favorably by outside forces leaves a stew ripe for conflict.
Translation: It's not the mix that causes conflict, but the stirring of them up over a simmering fire which eventually might explode.
Go figure.
