Lenin adopted Marxism as a young man, and never wavered in his ideological commitment. It was not unusual for members of his class to adopt revolutionary ideologies in the repressive climate of Tsarist Russia, and Marxism, named for the German thinker Karl Marx, was the most influential of these ideologies. Marx claimed that in an industrial society the triumph of the middle class, or bourgeoisie, eventually gave way to the rise of a proletariat, or laboring class: the bourgeoisie possessed wealth only at the expense of an ever-more-impoverished working class, leading to a revolution and the establishment of the "dictatorship of the proletariat." This rule by the proletariat set the basis for a utopia in which all class distinctions would disappear, as would poverty itself. Marxism had an especially powerful appeal for those living in the stratified and desperately poor society that characterized turn-of-the-century Russia: it offered the promise of a better world in a rhetoric of scientific certainty. But Marxism attracted Lenin not just as a Russian, but as an individual with his own personal history: his older brother, Alexander, had been a revolutionary, and was executed by the government for plotting to assassinate Tsar Alexander III. Shocked by this execution, Lenin vowed to work in his brother's footsteps; and Marxism provided the opportunity for him to take up a similar radicalism.

Put simply, the Bolsheviks represented the more radical of the Russian Marxists, and the Mensheviks the more moderate. But this does not mean that the Mensheviks were not a revolutionary party: rather, the two groups differed over what kind of revolution was to come, and thus which kind of party organization would best support this revolution. The Mensheviks, led by Y.O. Martov, wanted a broad-based party, with loose membership requirements, since they anticipated a broad-based, popular revolution. Lenin dismissed this vision: in his mind the revolution would be the work of a small group of committed revolutionaries, not a popular groundswell. Thus the issue that proved ultimately decisive was party membership and its requirements; when the issue came to a vote in 1903, the two groups parted ways, never to be reunited. Lenin's Bolsheviks became the tight cadre that he had argued for, and it was they, and not the Mensheviks, who would seize power in 1917.

Lenin was still in Switzerland when Nicholas II abdicated in March 1917, and so he played no role in the formation of the Provisional Government that took over from the Tsars. The fall of the monarchy had not come unexpectedly– Nicholas II's rule had been tottering for some time, and the populace had turned against him and his German wife; they were now ready for a revolt. But no one expected the government that followed this fall–the Provisional Government–to suffer from similar problems. Rather, it seemed, even to Bolsheviks such as Stalin and Kamenev, that the Provisional Government would last, at least for a time, and that the Bolsheviks were best served by settling into the role of an opposition party, rather than risk an all-out takeover. Only Lenin was able to see this Provisional Government's weakness, both while in Switzerland and then back in Russia. It was this vision that urged the Bolsheviks away from compromise and toward confrontation, leading to their seizure of power and rise to supremacy in November 1917. After that, it was his leadership during the crucial following months that kept his party firmly on the track to a revolution that few observers could have anticipated.