Following the First World War the international socialist movement was irreconcilably split into two hostile factions: on the one side, the social democrats, who broadly supported their national governments during the conflict; and on the other side Leninists and their allies who formed the new Communist Parties that were organised into the Third International, which was established in March 1919. However, during the Russian Civil War Lenin and Trotsky more firmly embraced the concept of national self-determination for tactical reasons. In the Third International the national question became a major bone of contention between mainstream Leninists and "left communists".