A stable country is a country that is primarily a country of X. What constitutes X may change over time, but it works fine if it is slow and gradual.
> But one of the legacies of colonialism is the Middle East and Africa being carved up in lines that didn’t match the underlying cultural groups.
If it is not gradual and you have Y and Z beside X, is when trouble start. Middle East and lot of African countries is the example of this. Those people where unable to live together (too many cultural differences) in their home country, why should be different in Europe, where cultural differences will be even higher ?