The Bolsheviks were originally part of the Russian Social Democratic Labor Party (RSDLP), which followed the teachings of Karl Marx. They became a separate entity at the Second Party Congress in 1903 when the majority ('bolshinstvo' means majority in Russian) split off after a critical vote. During the Russian Revolution the Bolsheviks took power, in October 1917, in St. Petersburg (known at the time as Petrograd). After winning the Russian Civil War they transformed into the Communist Party of the Soviet Union.
Vladimir Lenin was the first leader of the Bolsheviks. Later, in his exile, Leon Trotsky, who did not align himself with the Bolsheviks until August 1917, would use the term Bolshevism to describe the original aims that Lenin had in contrast with what the Communist Party had become under Joseph Stalin.
The dispute, which led to the split in the RSDLP, was over membership rules. Lenin wanted to limit membership to people who were actively committed to the party goals, who contributed money and time to the cause. The leader of the other faction, Julius Martov, agreed that the core members should consist of party activists but also believed that anyone who sympathized with the goals of communism should be allowed to join. Martov also believed in working with other socialist or anti-tsarist groups, but Lenin refused to do so except when a clear tactical advantage presented itself.
The Bolsheviks only played a minor role in the Russian Revolution of 1905. The Tsar retained power after that uprising but did allow for a national assembly (Duma) and a Russian Constitution. Lenin supported Bolshevik participation in the Duma, but many other members of the party, led by Alexander Bogdanov, wanted to pull their members out. Lenin undermined Bogdanov's authority and successfully ousted him from the Bolsheviks. Several attempts were made at reunifying the RSDLP, but by 1912 the Bolsheviks quit trying and declared themselves an independent party.
During World War I, Germans such as Max Warburg financed the Bolsheviks. The goal was to take the pressure off the Eastern front through a communist revolution. The hope was the Germans could then win the war on the Western front before the Americans became involved. The strategy was successful in taking pressure off the Eastern front but was too late to make enough of a difference. Winston Churchill saw the danger the Bolsheviks represented and argued unsuccessfully for Britain to keep fighting the Bolsheviks until they were defeated.
On February 17, 1917, Tsar Nicholas II abdicated his throne and a Provisional Government was installed. Lenin was unhappy with the lack of progress the Provisional Government made, and on October 24, 1917, Bolshevik forces arrested the members of the Provisional Government. Despite losing elections a month later, the Bolsheviks still took control. Three years of civil war ensued, with the Bolshevik Red Army not only fighting the Russian White Army but also the Allies. When World War I ended and the Allies withdrew their troops, the Bolsheviks won the civil war and consolidated their power, remaining in control of the Soviet Union for seven decades.
Numerous reasons for Western aid to the Bolsheviks have been advanced – such as the hope of Western leaders that Russia would fight Germany. But given Western elites' support for authoritarianism to justify further authoritarian crackdowns at home, one could make the argument – simply enough – that the West supported the Bolsheviks simply to introduce a large state to totalitarianism. Under this reasoning, the eventual "Cold War" was a welcome development, benefitting the Western military-industrial complex and those Western elites who benefit from such corporatist evolutions.