12 A much more serious and lamentable evil than these threats of external aggression is the internal discord which menaces the welfare not only of nations but of human society itself. In the first place, we must take cognizance of the war between the classes, a chronic and mortal disease of present-day society, which like a cancer is eating away the vital forces of the social fabric, labor, industry, the arts, commerce, agriculture - everything in fact which contributes to public and private welfare and to national prosperity. This conflict seems to resist every solution and grows worse because those who are never satisfied with the amount of their wealth contend with those who hold on most tenaciously to the riches which they have already acquired, while to both classes there is common the desire to rule the other and to assume control of the other's possessions. From this class war there result frequent interruptions of work, the causes for which most often can be laid to mutual provocations. There result, too, revolutions, riots, and forcible repression of one side or other by the government, all of which cannot but end in general discontent and in grave damage to the common welfare. To these evils we must add the contests between political parties, many of which struggles do not originate in a real difference of opinion concerning the public good or in a laudable and disinterested search for what would best promote the common welfare, but in the desire for power and for the protection of some private interest which inevitably result in injury to the citizens as a whole. From this course there often arise robberies of what belongs rightly to the people, and even conspiracies against and attacks on the supreme authority of the state, as well as on its representatives. These political struggles also beget threats of popular action and, at times, eventuate in open rebellion and other disorders which are all the more deplorable and harmful since they come from a public to whom it has been given, in our modern democratic states, to participate in very large measure in public life and in the affairs of government. Now, these different forms of government are not of themselves contrary to the principles of the Catholic Faith, which can easily be reconciled with any reasonable and just system of government. Such governments, however, are the most exposed to the danger of being overthrown by one faction or another.
Source: Ubi Arcano Dei Consilio (Vatican.va)