In 1909 the Social Democratic Union 'Proletarian' that had been expelled from the Narrow Socialists, merged into the Broad Socialist party. At the Zimmerwald Conference, where the unraveling of the coalition between revolutionary socialists and reformist socialists in the Second International began, the party supported the so-called Zimmerwald Left. It opposed World War I and was sympathetic to the October Revolution in Russia. Under the influence of the Bolsheviks the "narrow socialists" accepted the ideas of Leninism. Under Blagoev's leadership, the party applied to join the Communist International in 1919. Upon joining the Comintern the party was reorganised as the "Bulgarian Communist Party".