[thesash]

Well, look at the centuries of violent war that preceded most modern, stable nation-states. Europe was pretty much in a constant state of conflict from the fall of the Roman Empire until WWI.

Nationalism is also an interesting phenomenon, in that over time it tends to transcend ethnic groups, but only once the thread of common identity has established itself. Take a look, for instance, at the post WWI Middle East, where the imposition of arbitrary zones of influence (the origins of many modern states) disrupted a budding pan-Arab nationalism in favor of keeping a balance of influence between the colonial powers.

The United States is a completely different case in that the nationalism that took root here was based on a shared identity among immigrants of (relatively speaking) diverse ethnic groups-- but even so, our early history had no shortage of atrocities committed agains native populations, as well as conflict among the constituent religious and ethnic groups that comprised the nation.

That being said, nation states seem to be a very stable and agreeable governance paradigm once they become established, but a shared identity amongst diverse groups (ethnic or otherwise) takes time to develop, and as history has demonstrated, it certainly can't be imposed from the outside.