Orwell wrote Animal Farm to illustrate the way Stalinism had betrayed the ideals of the socialist revolution in the Soviet Union. As he put it, "I thought of exposing the Soviet myth in a story that could be easily understood by almost anyone and which could be easily translated into other languages."
Though popular today, when alive Orwell managed to alienate people on both the left and the right. Although he was a Democratic Socialist, socialists condemned him for betraying the revolution in pointing out its flaws (as he did in Animal Farm) and rightwing people condemned him for his socialism.
Orwell called it as he saw it, and like other leftwing intellectuals, was dismayed at Stalin, a person he depicts as Napoleon the pig in the novel. Like Stalin, Napoleon has show trials in which animals, such as the three poor, not very intelligent hens "confess" to so-called crimes and are executed. Like Stalin, Napoleon runs his rival Snowball (Trotsky) out of the country. Also like Stalin, Napoleon ends up signing a pact with the enemy. In Stalin's case, it was Hitler; in Napoleon's case Farmer Jones.
We may not remember the distinct historical parallels that Orwell references, but the warning against an idealistic rebellion turning into a tyranny is still being heeded.