As George Orwell's Animal Farm progresses, two boars named Napoleon and Snowball emerge as the leaders of the animals' rebellion. Both animals have very different leadership styles, however, with Napoleon being more of "the strong, silent type", whereas Snowball is a very persuasive idealist.
By Chapter 3, we are given a very strong indication of their future rift as Orwell tells us that the two animals never agreed upon anything and constantly opposed each other's viewpoints. Additionally, Napoleon shows no interest in the various committees that Snowball was so keen on creating and organizing.
In Chapter 5, however, we learn that "...of all their controversies, none was so bitter as the one that took place over the windmill." Snowball's plan to build a windmill, which would provide all manner of advantages for the farm, was opposed by Napoleon, who, once he saw the plans Snowball had drawn, "urinated over the plans".
This conflict over the windmill eventually reaches such a height that Napoleon resorts to force to drive Snowball away from the farm:
...the dogs were gaining on him again. One of them all but closed his jaws on Snowball's tail, but Snowball whisked it free just in time. Then he put on an extra spurt and, with a few inches to spare, slipped through a hole in the hedge and was seen no more.
After these events, Snowball's previous contributions to the Rebellion were discounted and undermined by Napoleon and any bad event that happened on the farm was blamed upon Snowball.
Ah, Napoleon and Snowball! This is a reference to Lenin and Trotsky. Lenin eventually took over everything in Russia and Trotsky was exiled. Things didn't end well for Trotsky, who was really, in my mind, the backbone to the Russian Revolution. Orwell was making an allusion to Lenin and Trotsky through Napoleon and Snowball. Pretty much everything that happened to Snowball in the book happened to Trotsky.