Huge amount of problems are created by intentional intrusion, forced or by own will of intrudes, of aliens with culture/civilisation without intent of integration into society.

All that begun with spreading of Jews around the world. Being alien to locals they do not allow assimilation of their own along with penetrating everywhere they could. Keeping their tribe intact and playing by their own rules, ignoring the rules of hosts, they had an advantage over local population. Everything else, from african slavery to modern muslim invasion in Europe would have been impossible without the ground thoroughly prepared by Jewish infiltrators.

As humans we are different, we have different civilisations developed in different conditions and with different history. Mindless forced mixing of different civilisations have hiuge consequences. Two countries with different culture could be fine neighbours with fair trade and relations, but could easily turn into mess if you forcefully mix their population.