A lot of times, people in the developed world claim diversity is causing an increase in crime in their nation-state. I have to wonder if this is even partly true seeing the tribalist crimes being committed daily in Africa, a continent with very few nation-states. Seeing that lands with one race/ethnicity have very low crime rates in the stats, there's even more to back the argument that diversity increases crime. But I'd still insist that it's a lack of national unity or so-called "social cohesion" more than just the concept of "foreigners have moved into the neighbourhood". Lands with no common identity tend to suffer from crime caused by "diversity" more frequently because there's no moral identity, people have different concepts of what is wrong & right.
Of the eight countries with the lowest crime rates in the world, five can be considered ethnostates.
The solution:
Quiet often we want to forcefully integrate minorities into the national society & it often works but there's always going to be something lacking for the outsider either in himself or his society that won't allow him to fully integrate in a community that is not of his race or ethnicity so, from my point of view, this can only be remedied by seperate but equal societies. This means, minorities being given their own institutions (like schools, neighbourhoods, sport's academies etc.) but being treated as equal citizens within the land. If this is maintained, in time, it creates a seamless integration with the land & the minorities/foreign communities - eventually - feel part of a land by identifying with a community in a land & growing up as part of a community than a lone foreigner in an established community. Creating separate institutions for minority groups should be carefully done & cultural differences be taken into account like race, language & religion. Allowing minority groups to fully express their identity in their own neighbourhoods, in a foreign land could be the difference between having anti-discrimination riots & minority communities contributing positively in a country.
Refugee camps must be established, minority groups with significant populations must be allowed to make their own institutions in their own communities without interference. This is not to say these minority populations will be under their own law in their own neighbourhoods but will simply be given freedom of expression within the laws of the land. National school syllabus, the national language & national law should still apply in the neighbourhoods of these minorities. Their local police could be members of their own ethnicity & local businesses could be of their own doing.