Separate development is a requirement for white survival.

More simply, throughout most of history, "ethnostate" and "state" pretty much meant the same thing. A state is a people and their territorial possessions taken as a whole; or should I say that is the "natural" definition of a state, and that ever since such was made, there have been efforts - undertaken largely by governments and administrations - to move the definition to "a government, its borders, and its population".

Moving the locus of national identity from the intangible gestalt of "the people" and into some government department is of great value to government - territory can be bought, sold, traded, or conquered, and people can likewise be exchanged freely via immigration patterns, leaving the government itself as the sole defining component of the nation.

Now, of course, history also shows us numerous examples of multi-ethnic nations - most large empires became such in the course of their expansion, and almost all the nations that exist today were formed from the agglomeration of tribal precursors. However, looking at history, we can also see which of these "diverse" nations survived, which collapsed, and the reasons why.

In every successful multi-ethnic society, peoples are mostly, if not entirely, divided by geography. It may be an enforced separation, mandated by the government, or it may be an organic separation, created by people's own patterns of behavior; but where multi-ethnic states work, they do so by giving their component peoples their own homelands within the greater nation, and often by letting them be governed by one of their own kind.

Furthermore, if we look at the most successful multi-ethnic empires - including China (Lifetime Achievement Award), Britain (Endless Sunshine Medal) and the United States (Geopolitical Speed Record), all of them successfully promoted a national "overculture" that allowed all of their internal ethnic groups to share in the national identity, at least to some degree. (Rome also did this, and then went too far and fucked itself. Its playbook seems to have been a best-seller.)

When it works, this allows them to pull "me and my brother against my cousin; me, my brother, and my cousin against the world" type stuff on the global stage. This allowed them to avoid the main lethal flaw of mixed-population societies - showing diversity in the face of the enemy. But for such a thing to work, the national overculture must not only be strong, but must also be seen and felt to encompass its constituent cultures without restricting them.

Preconditions for such a thing might be said to be internal stability, sub-cultural cross-compatibility, and personal flexibility.

The first - internal stability - is taken as a precondition on the principle of immediacy. It's the whole "A house divided against itself is a bad house, a very bad house, no roofing tiles!" thing. The issue is that each group may well accept the overculture, and define it for themselves as being explicitly exclusive of those people they don't like (See: US Politics.). This can create a feedback loop of internal strife, until someone breaks out the molotovs and we have War: Civil Style.

The second - sub-cultural cross-compatibility - is something that to most of us here is probably well understood enough. The more compatible, or similar, the constituent cultures are, the more easily they will group under the umbrella of the overculture without getting peeved at the smell of each other's cologne.

Brits, Germans, and Frenchmen might all be natural rivals and occasional enemies in their homeland, but they'd all get along easier with each other in isolation than they would with a Chinaman; and they'd all get along easier with a Chinaman than they would with an African or an Arab, and so on and so forth. This is largely a function of the cultures themselves, and how much overlap there is in their underlying value systems, which in turn derives in part from the genetically guided predilections of their populations. We know this argument already.

Lastly, personal flexibility, is less of a formal precondition and more of a balancing factor that can, to some degree, compensate for minor shortcomings in the others. Defined as basically the "live and let live"-ness of a population, its primary sources are either a genetics-to-culture feedback loop (which seems to be the standard in the occident), or some manner of religious, spiritual, or philosophical tradition (which, conversely, is the typical standard in the orient), though there is likely elements of both at play in most cases.

Without these conditions, it will be functionally impossible to institute a single overculture. Lacking something like that, a nation best hopes that its constituent peoples either truly see one another as cousins, or else there's going to be a fight - it's just a matter of when, and who gets the bruises.