Complex sentence: Before 1914, Lenin's views were largely in accordance with mainstream European Marxist orthodoxy, although the subsequent development of Leninism revised this orthodoxy and introduced innovations in Marxist thought.
Split into simpler sentences: Before 1914, Lenin's views were largely in accordance with mainstream European Marxist orthodoxy. Thus, as Leninism developed, Lenin revised the established Marxist orthodoxy and introduced innovations in Marxist thought.