That is the Decline of Assabiyyah or Social Cohesion.

In multi-Ethnic nations, the solution is to forcibly move different ethnic groups with each other, as noted by Singapore's LKY, to prevent Diasporaic cultures from forming. But the Belgians didn't have to be a multi-ethnic society, nor do the natives seem to tolerate that. It really depends on the society and its willingness to do so.