 After his exile, he moved to Western Europe, where he became a prominent theorist in the Marxist Russian Social Democratic Labour Party (RSDLP).In 1903, he took a key role in a RSDLP ideological split, leading the Bolshevik faction against Julius Martov's Mensheviks.Encouraging insurrection during Russia's failed Revolution of 1905, he later campaigned for the First World War to be transformed into a Europe-wide proletarian revolution, which as a Marxist he believed would cause the overthrow of capitalism and its replacement with socialism.