What is Animal Farm an allegory for?
Animal Farm is an allegory for the Russian/Bolshevik Revolution in particular, and for the rise of totalitarian dictatorships based on communist principles in general. Orwell was a democratic socialist, and like many European (and American) leftists during the Great Depression, he viewed the Bolshevik Revolution and the establishment of a communist state with great interest and even hope. But as the decade wore on, he found himself deeply disillusioned as news of the brutality of Stalin's regime leaked out, and as he witnessed firsthand the death and destruction of the Spanish Civil War. Orwell's contempt for the Stalinist regime became complete when Stalin signed a non-aggression pact with Hitler in 1939. He wrote the book a few years later, and published it at the end of the war, in 1945.
So Animal Farm is replete with allegorical figures and events. Old Major, whose impassioned speech provides the basis for the ideas of Animalism, is meant to evoke Karl Marx. Napoleon, who rises to power by combining brutality and manipulation of the truth, is suggestive of Joseph Stalin. Snowball, his rival, is comparable to Leon Trotsky, who fled the Soviet Union and was eventually assassinated by Stalin's henchmen. Like Stalin's bloody purges, there are massacres of animals who are thought to be disloyal, and the windmill project is reminiscent of Stalin's program of forced industrialization. The central theme of this allegory is that power, even when wielded in the name of equality and economic justice, corrupts, and the people (animals) suffer.