How did the Russian Revolution of 1917 divide communist parties from social democratic parties?

The Russian Revolution of 1917 served as a definitive catalyst for the split between communist and social democratic parties, formalizing a divide that had been widening since the outbreak of World War I.12

Before the revolution, the socialist movement was already fractured by debates over revisionism and national loyalty. When the war began, most socialist parties supported their respective national governments, abandoning international class solidarity. Vladimir Lenin and his Bolshevik faction rejected this nationalism, advocating instead for a transnational class war.12

Following the Bolshevik seizure of power in 1917, the divide became absolute:

• Institutionalization of the Split: In 1919, Lenin established the Third International (Comintern) to promote world revolution. Membership required parties to adopt a disciplined, Soviet-style structure and explicitly expel moderate socialists and pacifists.2
• Political Suppression: Within Russia, the Bolsheviks moved to consolidate power by suppressing non-Bolshevik socialist groups, such as the Mensheviks and Socialist Revolutionaries, through the use of the Cheka and the implementation of "Red terror".34
• Ideological Divergence: Communists increasingly defined themselves by their commitment to revolutionary overthrow and the dictatorship of the proletariat, while social democrats moved toward democracy and peaceful, gradual political change.15