Bolshevism was one of the two main branches of socialism in Russia from 1903 until Lenin strengthened the Bolshevik movement in the Civil War of 1918-20. The Russian Social Democratic Labor Party formed in 1898 in a secret conclave in Minsk was based on Marxist principles. Later, when Lenin's group gained a majority at the subsequent party congress in Brussels and then in London they were termed as Bolsheviki or the members of the majority. Lenin's opponents were called Mensheviki or the members of the minority.

The Russian Revolution of 1905 was a concerted movement of all the revolutionary groups. In 1912, the Bolsheviks and Mensheviks formed distinct parties and during World War I the former hoped for the defeat of the czarist regime and tried to give the international conflict the colors of a civil war. During the Russian Revolution of 1917, the Mensheviks joined the Kerensky provisional government. The Bolsheviks supported the government initially and were considering a merger with the Mensheviks, when Lenin returned from exile and expressed his opposition. His views were accepted and the Bolsheviks grew in popularity among the common people. The Bolshevik group increased from some tens of thousands to over a quarter of a million by October. They managed to gain a majority in most of the major Soviets and then overthrew the government in the October Revolution.

This was the Bolshevik group that formed the new government and then became the Communist Party that ruled the USSR until the end of the Cold War, though the original party went through many changes in name and composition. Most of the earlier major revolutionary leaders lost their importance and were marginalized. The Mensheviks tried their best to organize an opposition group but failed and in the early 1920's, were totally eliminated from the political scene.