By the first years of the twentieth century, democratic revisionists had developed a powerful critique of the international socialist movement's reigning Marxist orthodoxy and laid the foundations for an ideological alternative. But it would take the vast changes unleashed by the First World War for democratic revisionism to blossom into a movement of its own. The key steps in this transformation were the open rejection of the twin pillars of orthodox Marxism – class struggle and historical materialism – and the embrace of their antitheses – cross-class cooperation and the primacy of politics.

The first pillar suffered a critical blow with the outbreak of the war. Socialist parties across the continent abandoned their suspicion of bourgeois parties and institutions and threw their support behind the states they had hitherto pledged to destroy. Even the German SPD, the International's largest party and the standard-bearer of Marxist orthodoxy, pledged itself to the defense of the Vaterland and quickly voted to authorize war credits. As the Russian Menshevik leader Paul Axelrod reported, “the news [that the SPD had voted to support the war] was a terrible, stunning blow. It appeared as if an earthquake had overcome the international proletariat. The tremendous authority of [the SPD] disappeared with one stroke.” In France, the Socialists not only joined with other groups in a union sacrèe to defend the patrie but, putting aside years of controversy, also sent two of their most prominent members – Jules Guesde and Marcel Sembet – to join the government.