https://iclfi.org/pubs/wv/1180/quote
Hundreds of thousands in the U.S. have demonstrated in opposition to Israel’s genocidal war against the Palestinians. We are for an end to this carnage. But this movement is a political bloc with liberals in the Democratic Party that militarily and politically backs the Zionist state to the hilt. Palestinian liberation can be achieved only through struggle against imperialism and breaking from its opportunist apologists in the labor movement. As Lenin explained at the outbreak of World War I in a polemic against the proponents of parliamentary reformism (such as Millerand and Bernstein), effecting such a split is central to the fight for revolutionary leadership.
Social-chauvinism and opportunism are the same in their political essence; class collaboration, repudiation of the proletarian dictatorship, rejection of revolutionary action, obeisance to bourgeois legality, non-confidence in the proletariat, and confidence in the bourgeoisie. The political ideas are identical, and so is the political content of their tactics. Social-chauvinism is the direct continuation and consummation of Millerandism, Bernsteinism, and British liberal-labour policies, their sum, their total, their highest achievement....
Unity with opportunism means unity between the proletariat and its national bourgeoisie, i.e., submission to the latter, a split in the international revolutionary working class. We do not say that an immediate split with the opportunists in all countries is desirable, or even possible at present; we do say that such a split has come to a head, that it has become inevitable, is progressive in nature, and necessary to the revolutionary struggle of the proletariat, and that history, having turned away from “peaceful” capitalism towards imperialism, has thereby turned towards such a split.
—V.I. Lenin, “Opportunism, and the Collapse of the Second International” (end of 1915)
