Well, unfortunately, pre-1950s Europe is the closest thing we have to the hypothetical and much desired White American state. Because from the very beginning, the US has had major contact with people of other races.
Whether it was 20-25% of the total population being slaves in colonial times, to genociding Native Americans, to taking half of Mexico, to the arrival of millions of immigrants from Latin America and Asia, there has been constant contact with nonwhites. So the US isn't a good example.
Europe as a whole, on the other hand, was 95%+ white for centuries, probably millennia, so they're far more similar. Much like the hypothetical US state, they were also racially white, with lots of ethnic/cultural variety.
Obviously, tons of political organs are in place now that would likely be factors in (the much-maligned EU, for example) keeping peace, but this isn't meant to be an all-encompassing analysis, just one based on ethnic groups.