In the words of John Glubb, I think this is the problem of multiculturalism:
In the age of the first outburst and the subsequent Age of Conquests, the race is normally ethnically more or less homogeneous. This state of affairs facilitates a feeling of solidarity and comradeship. But in the Ages of Commerce and Affluence, every type of foreigner floods into the great city, the streets of which are reputed to be paved with gold. As, in most cases, this great city is also the capital of the empire, the cosmopolitan crowd at the seat of empire exercises a political influence greatly in excess of its relative numbers.
Second- or third-generation foreign immigrants may appear outwardly to be entirely assimilated, but they often constitute a weakness in two directions. First, their basic human nature often differs from that of the original imperial stock. If the earlier imperial race was stubborn and slowmoving, the immigrants might come from more emotional races, thereby introducing cracks and schisms into the national policies, even if all were equally loyal.
Second, while the nation is still affluent, all the diverse races may appear equally loyal. But in an acute emergency, the immigrants will often be less willing to sacrifice their lives and their property than will be the original descendants of the founder race.
Third, the immigrants are liable to form communities of their own, protecting primarily their own interests, and only in the second degree that of the nation as a whole.
Fourth, many of the foreign immigrants will probably belong to races originally conquered by and absorbed into the empire. While the empire is enjoying its High Noon of prosperity, all these people are proud and glad to be imperial citizens. But when decline sets in, it is extraordinary how the memory of ancient wars, perhaps centuries before, is suddenly revived, and local or provincial movements appear demanding secession or independence.