Pacem in Terris §94

⁹⁴ A special instance of this clash of interests is furnished by that political trend (which since the nineteenth century has become widespread throughout the world and has gained in strength) as a result of which men of similar ethnic background are anxious for political autonomy and unification into a single nation. For many reasons this cannot always be effected, and consequently minority peoples are often obliged to live within the territories of a nation of a different ethnic origin. This situation gives rise to serious problems.