What happens to Snowball during the meeting about the windmill? What events in Soviet history does this scene suggest?

In Chapter Five of Animal Farm, the debates about the windmill reach their climax as Snowball makes his case to the rest of the animals. In his rousing speech, Snowball emphasises the potential benefits of the windmill:

He painted a picture of Animal Farm as it might be when sordid labour was lifted from the animals' backs.

This rosy image succeeds in convincing the animals that Snowball should build the windmill. But, sensing a threat to his dominance, Napoleon reacts with unprecedented brutality and cruelty: he utters a "high-pitched whimper" which alerts his pack of dogs. They immediately enter the barn and run directly at the defenceless Snowball, who is saved only by his quick reflexes. Snowball runs as fast he can across the pasture and out of the farm, with the dogs close on his tail. Luckily, Snowball is able to slip through a hole in the fence, but is never again seen on Animal Farm. 

To put this scene into historical context, it is reminiscent of Leon Trotsky's exile from the Soviet Union in 1928. Like Snowball, Trotsky criticised some of Stalin's policies, especially those relating to the economy and to the suppression of democracy, and was first thrown out of the party, then out of Moscow and out of the country completely. In this scene, therefore, Orwell uses Napoleon to represent Stalin's unexpected actions against Trotsky. 

You can find out more about this moment in history in the reference link provided.