As Lenin grew sick in the early 1920s, serious tension started to mount between Joseph Stalin (Napoleon) and Leon Trotsky (Snowball). Trotsky had already been critical of Stalin’s war record, but what really set them apart was that Trotsky wanted to continue to spread the Revolution abroad, whereas Stalin wanted to focus on building communism in the territories Russia had already acquired.
Stalin used his position as General Secretary of the Communist Party (an appointment Lenin later regretted) to build a coalition against Trotsky, and essentially make him a mute political force. Following Lenin's death, Trotsky was forced into exile, leaving Stalin in complete control by about 1928.
In Animal Farm, the divide between Stalin and Trotsky (Napoleon and Snowball) is represented by the argument over the windmill. We learn that Napoleon despises the idea – at one point he "urinated over the plans and walked out without a word" – and, in general, "the whole farm was deeply divided on the subject of the windmill" (5.10, 11). The windmill is, in many ways, the perfect symbol for the decision about whether or not to expand communism. It retains its links with the story of Don Quixote, where it comes to represent a fantastic and probably unachievable dream. In this case, the dream is worldwide communist revolution.
As soon as Snowball is off the farm, Napoleon begins to consolidate power for himself, as Stalin did in Russia after he exiled Trotsky. We find that he has reared the nine pups and made them his guard-dogs (equivalent to Stalin’s secret police, the NKVD), and that he has made Squealer his right-hand man. The animals are vaguely troubled by all of this, and "several of them would have protested if they could have found the right arguments" (5.17), but they do nothing.