...a survey would show that whereas in 1900 there were many states in Europe without a single overwhelmingly dominant nationality, by 2007 there were only two, and one of those, Belgium, was close to breaking up. Aside from Switzerland, in other words -- where the domestic ethnic balance of power is protected by strict citizenship laws -- in Europe the "separatist project" has not so much vanished as triumphed. Far from having been superannuated in 1945, in many respects ethnonationalism was at its apogee in the years immediately after World War II. European stability during the Cold War era was in fact due partly to the widespread fulfillment of the ethnonationalist project. And since the end of the Cold War, ethnonationalism has continued to reshape European borders.
One could argue that Europe has been so harmonious since World War II not because of the failure of ethnic nationalism but because of its success, which removed some of the greatest sources of conflict both within and between countries. The fact that ethnic and state boundaries now largely coincide has meant that there are fewer disputes over borders or expatriate communities, leading to the most stable territorial configuration in European history.This is, of course, quite different from how Europe pictures itself these days. The standard belief is that a pan-European consciousness is growing, and nationalism fading, except in retrograde parties which hate outsiders. That the ability of the EU to come together may actually be founded on its individual components first becoming separate is not such a popular idea just now.
It was very much the American idea at first, of disparate colonies coming together. And certainly, an American identity has been established. As federalism disappears (or so I hear), American unity may fragment as well. Union may require separateness.
Note that since WWII, and accelerating after the breakup of the USSR, much of nation building has involved people returning ethnic groups, mostly Germans, back to their countries of "origin," even had they lived there for generations. This was often not expulsion but volunteering, e.g. Germans and many Hungarians leaving Romania; Germans moving west from Poland and Poles West from Belarus;
(HT: Byzantine Calvinist)