weird because we've been relatively paceful only in the first world and only in the last 30 or so years. jugoslavia was just torn in an ethnic war; spain, ireland and russian client state were host to terrorost movement up until a couple generation ago, with many issue yet unresolved and likely to resurface. before the first world war there were a host of proxy wars around africa and middle east, class cleansing in the east, before that full on wars in the americas, before that france england and germany were in a state of constant skirmishes, with spain under siege by the moors and italy cities changing state flag every couple of generations. china was in warring state mode too until unification, then class warfare, then religious warfare, then the great step ahead.

humans have been a bunch of assholes, we living in a niche of limited peace on a few locations shouldn't give us the peace of mind that everyone else understand the value of integration. note that a person or a family by itself is likely to integrate, it's significant migration that end up into more or less pronounced isolationism.

again, just have a look at all the chinatown and italian quarters. the actual border may be fuzzy, but the sense of identity is definitely there. and while wwii was an exceptional time, smaller conflict are still very present and the failure integration models are popping up every day stronger.