"I have already said that between Marx and Engels on the one hand, and Lenin, on the other, there lies a whole period of domination of the opportunism of the Second International. For the sake of exactitude I must add that it is not the formal domination of opportunism I have in mind, but only its actual domination. Formally, the Second International was headed by "faithful" Marxists, by the "orthodox"--Kautsky and others. Actually, however, the main work of the Second International followed the line of opportunism. The opportunists adapted themselves to the bourgeois because of their adaptive, petty-bourgeois nature; the "orthodox," in their turn, adapted themselves to the opportunists in order to "preserve unity" with them, in the interests of "peace within the party." Thus the link between the policy of the bourgeois and the policy of the "orthodox" was closed, and, as a result, opportunism reigned supreme."
J.V. Stalin | Foundations of Leninism