When the old monarchical regimes were still around, one of the main ways that emerging nation-states differentiated themselves was by displaying a strong opposition to national pluralism — which is to say creating states composed of many different ethnic or religious groups.
The wars of the reformation in large part created the backlash against pluralism — England expelled or harried its papists, France eventually eliminated their Huguenots, and the counter-Reformation purged non-Catholics from Catholic countries. All of this happened after some extended experiments in pluralism and toleration, most of which resulted in too many elite power struggles for the outlook to be maintained.
In many areas — particularly under the Holy Roman Empire — ethnic, cultural, and linguistic pluralism within the empire could be maintained so long as the provinces remained loyal to the emperor and the pope both. Some measure of religious pluralism within the Catholic Church also made it so that there were many different expressions of religious worship even under what was nominally the same religion.
Monarchs and nobles were often ethnically and linguistically distinct from their subjects — but with, generally speaking, a smaller cultural gulf than we would expect from a ‘multicultural’ society of today. The European order developed out of the Roman order with its demonstrated success over a thousand years of absorbing subsidiary cultures into a relatively cohesive continental empire.
From the early modern period until the end of World War II, emerging nation-states tended to reject the pluralism of the old regime. The countless languages and dialects of provinces were melted into official-national-languages. Religions declined in importance, while ideologies promoted by the press tended to take their place.
After World War I, the old regimes were all dead. But that didn’t stop the conflicts — the new regimes of self-determined democratic nation-states then got at one another’s throats for another apocalyptic conflict barely a generation after the previous one.
After World War II, the ideology-religion of humanism and human rights came to be the official belief of the global elite, which had come to be dominated by the United States. Humanism is the belief that all people are spiritually equal, and that that spiritual equality obligates all political leaders to craft policies that bring material conditions for people in line with that essential spiritual equality.
States can only maintain so much pluralism while maintaining coherent enough to administer. When there is a great deal of ethnic, linguistic, and cultural diversity within a state, the religion must be unified, or else the difference in outlooks among the populace will lead them into permanent conflicts with their neighbors.
Similarly, when religious pluralism is permitted, little diversity in other matters can be permitted without risking serious strife and civil war. The first lasting government in the United States — a state with more religious pluralism than most others formed at the time — didn’t make it a century without a bloody civil war, demonstrating the dangers involved in religious toleration.
So in modern states, there tends to be minimal religious diversity permitted — all must bow down to humanism and a raft of other progressive beliefs — but there is maximal ethnic diversity mandated, because the latter policy is intended to demonstrate an absolute commitment to the doctrines of humanism. If leaders believe in humanism, then they will avoid enacting policies that might hint that they don’t believe that all humans from everywhere are equal in value.
Postwar nation-states take the principle of unlimited ethnic diversity in order to demonstrate a commitment to humanist values. Those humanist values have tended to be supported by even ‘realist’ thinkers and politicians not so much out of sincere belief but out of fear that the failure of that religious system would lead to a replay of World War II and the serious human costs that ensued. One of the chief reasons as to why World War I and II were so severe was that a Europe fragmented into mutually antagonistic ethnic nations proved to be unable to maintain sufficient common interests to avoid wars of near-annihilation which devastated the populations and elite classes of most of the belligerents involved.
Postwar elites find themselves in a serious bind. The hastily constructed godless religion of humanism is insupportable and results in serious instability both within nations and between nations. It encourages maladaptive behavior on a grand scale, and nature tends to correct such behaviors with a harsh hand.
These postwar elites have been trained into believing a false choice — that it’s either the noble lie of humanism, or global thermonuclear annihilation. This is the chief reason as to why this humanism is so resilient to all critiques — whenever it even comes to be slightly threatened by alternative beliefs, leaders feel a responsibility to shore it up, because they believe the alternative is so dire.