Orwell uses the allegory, Animal Farm, to present the story of The Russian Revolution and essentially express his opinions on the matter. By plainly exposing the unjust and corrupt system that is communism, Orwell is ultimately presenting his pessimistic view of human nature.
It is evident through the text that Orwell believes that in theory everybody wants equality, hence the concept of communism, yet it is in our nature as human beings to seek power. This can be shown in the text when the pigs initiate to overthrow Farmer Jones, so that all animals are henceforth equal. Yet as soon as the pigs assume authority, history repeats as they sink into the old habits of Farmer Jones, by exploiting the animals, cutting rations, increasing labour, and in effect creating an imbalance of equality.
This obvious hypocrisy and corrupt system gets so far out of hand that the situation the animals are in is indistinguishable from the condition they were in back when Farmer Jones ruled them. The pigs become comparable to Farmer Jones to the extent that one might consider them interchangeable.
From this example, we can conclude that it is in the nature of humans to seek power and authority, and many may go as far as to exploit those weaker than them, or disregard what is right and just to obtain power. One also may suggest that because people will always be power-driven, it is natural that history will repeat itself.
When Animal Farm was written, Orwell would have had to consider the level of censorship the government applied to any opinionated literature of a controversial political topic. This is why it was necessary for Orwell use an allegory fable to expose the serious issues behind the Russian Revolution.
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