25 These unsuppressed desires, this inordinate love of the things of the world, are precisely the source of all international misunderstandings and rivalries, despite the fact that oftentimes men dare to maintain that acts prompted by such motives are excusable and even justifiable because, forsooth, they were performed for reasons of state or of the public good, or out of love for country. Patriotism - the stimulus of so many virtues and of so many noble acts of heroism when kept within the bounds of the law of Christ - becomes merely an occasion, an added incentive to grave injustice when true love of country is debased to the condition of an extreme nationalism, when we forget that all men are our brothers and members of the same great human family, that other nations have an equal right with us both to life and to prosperity, that it is never lawful nor even wise, to dissociate morality from the affairs of practical life, that, in the last analysis, it is "justice which exalteth a nation: but sin maketh nations miserable." ( Proverbs xiv, 34)
Source: Ubi Arcano Dei Consilio (Vatican.va)