What is the most important theme in Animal Farm?
The most important theme in Animal Farm is the inevitable move from cooperative socialism to military dictatorship. Since the book is based on the real-life events of Communist Russia, it draws on that history. For this theme, the movements of Napoleon and Snowball are important; while Snowball is an idealist, truly seeking to create a socialist utopia on the farm where every animal contributes equally, Napoleon sees immediately that he can slowly accumulate power until it is impossible to defy him.
They did not know which was more shocking -- the treachery of the animals who had leagued themselves with Snowball, or the cruel retribution they had just witnessed. In the old days there had often been scenes of bloodshed equally terrible, but it seemed to all of them that it was far worse now that it was happening among themselves.
(Orwell, Animal Farm, msxnet.org)
This theme is shown through the propaganda and subversion techniques used by Napoleon and his supporters. They turn the minds of the animals towards them with fear and deliberate misinformation. The result is a culture of suspicion and fear, where the animals are afraid that if they don't support Napoleon, the humans will return; they don't understand that they have swapped one ruler for another. In addition, they don't realize how much worse their lives are because of the propaganda of Squealer; Boxer famously states: "If Napoleon says it, it must be true," and it is this blind faith that leads the animals into a far worse situation.