I'm not sure what the point of separating alongside nationalistic lines would achieve? Nationalism isn't an objective quality, but a quality that's perceived subjectively by number of people.
You'll never be able to divide on nationalistic lines because they aren't real lines, people can always fall into either a no true Scotsman fallacy, and even if that is settled you'll probably have outliers that might have a mother of nationality A and father of nationality B. Which nation does the child go? Not to mention two nations will probably want to contest same resources, so that leads to further strife.
The issue facing this society is lack of social cohesion, not the issue of nation.
You'll never be able to divide on nationalistic lines because they aren't real lines, people can always fall into either a no true Scotsman fallacy, and even if that is settled you'll probably have outliers that might have a mother of nationality A and father of nationality B. Which nation does the child go? Not to mention two nations will probably want to contest same resources, so that leads to further strife.
The issue facing this society is lack of social cohesion, not the issue of nation.