The corrupting effect of power is one of the central themes of Animal Farm. At the beginning of the book, Old Major describes the oppression that the animals experience, and predicts that the day will come when they overthrow their human masters and build an equitable society. When the animals of Manor Farm drive off Jones, it appears that day has come. But we quickly see that the pigs, by virtue of their leadership of the revolution, quickly become corrupted by power. Napoleon continues to pay lip service to the principles of the revolution through most of the book, but his actions are far removed from the principles of Animalism. He and the other pigs begin to claim privileges for themselves, and eventuallly he uses the dogs to purge those who question his authority. Snowball is driven from the farm for dissent, and gradually, the pigs become more like the humans they fought to overthrow. The corruption of the principles of the revolution is illustrated by the changing Seven Commandments, which are perverted over the course of the book to the point where, at the end, they read only "ALL ANIMALS ARE EQUAL BUT SOME ANIMALS ARE MORE EQUAL THAN OTHERS." The book's final passage, when some of the animals witness the pigs arguing and playing cards with Pilkington and the other humans in the farmhouse, makes the corruption of power most clear:
Twelve voices were shouting in anger, and they were all alike. No question, now, what had happened to the faces of the pigs. The creatures outside looked from pig to man, and from man to pig, and from pig to man again; but already it was impossible to say which was which.