Literally almost every society except very small tribes are multi-ethnic; and an American racial abstraction doesn't really apply to the rest of human history.
That being said, many of the most successful states tended to be Empires of one kind or another, who had multi-ethnic societies, and even "multi-cultural" aspects to it.
This is not to say that multi-ethnicity brings strength. Just that it's typically a non-sequitur. The issue is whether or not these different ethnic groups that fall under a state's jursidiction consider that state legitimate, or are prepared to be unified as a "nation" under an imperial banner (like Islam, Britain, Ottomans, Germans, all did; and like the Hapsburgs failed to do). Yes purely dynastic rule, like the Hapsburgs, is possible over different peoples, but it's not exactly a hugely stable system since the population always regard their rulers as foreign in some way or another.
Ehh, I just don't think a multi-racial society will ever really work.
Literally almost every society except very small tribes are multi-ethnic; and an American racial abstraction doesn't really apply to the rest of human history.
That being said, many of the most successful states tended to be Empires of one kind or another, who had multi-ethnic societies, and even "multi-cultural" aspects to it.
This is not to say that multi-ethnicity brings strength. Just that it's typically a non-sequitur. The issue is whether or not these different ethnic groups that fall under a state's jursidiction consider that state legitimate, or are prepared to be unified as a "nation" under an imperial banner (like Islam, Britain, Ottomans, Germans, all did; and like the Hapsburgs failed to do). Yes purely dynastic rule, like the Hapsburgs, is possible over different peoples, but it's not exactly a hugely stable system since the population always regard their rulers as foreign in some way or another.