Animal Farm runs on pig fat and lies. By the end, "truth" has become so malleable that the animals can't even remember what actually happened. A lie is something that contradicts the pigs' agenda; a truth is something that supports it. Napoleon might try to justify the lies by thinking they're for the good of the cause, but his propaganda machines is still cranking out the myths. Is our narrator the only source of unbiased fact? Or could you say that Animal Farm is a kind of anti-communist propaganda of its own?
While the pigs are able to convince some of the animals of their lies, they ultimately can't convince the majority of animals.
The pigs' ability to alter the official record of the past is their most powerful tool of control.