14 From the same atheistic source, socialism also derives its choice of the means of action condemned in Rerum novarum, namely, class struggle. The Pope does not, of course, intend to condemn every possible form of social conflict. The Church is well aware that in the course of history conflicts of interest between different social groups inevitably arise, and that in the face of such conflicts Christians must often take a position, honestly and decisively. The Encyclical Laborem exercens moreover clearly recognized the positive role of conflict when it takes the form of a "struggle for social justice"; Quadragesimo anno had already stated that "if the class struggle abstains from enmities and mutual hatred, it gradually changes into an honest discussion of differences founded on a desire for justice". However, what is condemned in class struggle is the idea that conflict is not restrained by ethical or juridical considerations, or by respect for the dignity of others (and consequently of oneself); a reasonable compromise is thus excluded, and what is pursued is not the general good of society, but a partisan interest which replaces the common good and sets out to destroy whatever stands in its way. In a word, it is a question of transferring to the sphere of internal conflict between social groups the doctrine of "total war", which the militarism and imperialism of that time brought to bear on international relations. As a result of this doctrine, the search for a proper balance between the interests of the various nations was replaced by attempts to impose the absolute domination of one's own side through the destruction of the other side's capacity to resist, using every possible means, not excluding the use of lies, terror tactics against citizens, and weapons of utter destruction (which precisely in those years were beginning to be designed). Therefore class struggle in the Marxist sense and militarism have the same root, namely, atheism and contempt for the human person, which place the principle of force above that of reason and law.
Source: Centesimus Annus (Vatican.va)