John

The "problem" I think rests not with diversity or homogeneity, but what the ethnicities of a country seek. The Balkans had diversity as Yugoslavia, but broke apart because each group wanted homogeneity. Meanwhile from 1870-1945 France and Germany were internally stable as homgenenous countries but fought three wars with each other in a desire to gain control over each country's respective ethnicites in the disputed Alsace-Lorraine area. Countries seeking diversity on the other hand rarely cause external issues or have significant internal ones (admittedly an over simplification, but fairly valid), notably the United States on being diverse and seeking diversity or the Roman Empire, which originally made up of homogenous Romans came to value diversity in other people's accomplishments, notably the Greek's.

Basically a country can be homogenous, as long as it values or tolerates foreign diversity, as Germany and France do today, or diverse and likewise value diversity. But any country where the ethnicities within value homogenaity, will either shake itself apart as in the Balkans, or war with its neighbors over ethnic conclaves in foreign lands. So while diversity is not required, tolerating it should be encouraged, since tolerant neighbors are more often than not peaceful ones.