Old Major's last words are as follows: "And, above all, no animal must ever tyrannise over his own kind....No animal must ever kill any other animal. All animals are equal" (21-22). After the pigs gain control of Animal Farm, these basic principles of Animalism are out the window, especially the idea of equality.
In order to keep the peace and remain in control of the other animals, without raising any red flags about the changes they continued to institute, the pigs offered up many different explanations for why things were the way they were. However, these "explanations" were more correctly propaganda, with Squealer as the mouthpiece for Napoleon.
Once Snowball was run off the farm by Napoleon's trained assassins (the dogs), the animals' society became more of a dictatorship ruled by Napoleon than an experiment in Socialism. Napoleon first appointed "a special committee of pigs, presided over by himself," (58). His first order of business was to cancel the Sunday-morning debates.
This change upset the animals, but they didn't possess the skills necessary to protest, and the few (young porkers) who uttered disapproval were immediately silenced by the dogs: "Suddenly the dogs sitting round Napoleon let out deep, menacing growls, and the pigs fell silent and sat down again," (59). Following this early change, Squealer was dispatched to keep the peace.
His feigned clarification included details about the extra hours Napoleon was putting in and the enormous amount of responsibility he had taken on as leader. He assured the animals that Napoleon still believed all animals were equal before laying down the guilt (propaganda) and scaring the animals into submission. "But sometimes you might make the wrong decisions, comrades, and then where should we be?" he asks (59).
Other examples of the propaganda (lies) spread by Squealer include the declaration that Snowball was a traitor, and that the windmill had been Napoleon's own idea, even though he had seemed opposed to it when Snowball presented it. "Squealer spoke so persuasively, and the three dogs who happened to be with him growled so threateningly, that they accepted his explanation without further questions" (62).
As the commandments gradually changed on the wall of the big barn to allow the pigs to behave in ways that had been prohibited under the original list, some animals took notice. "And Squealer, who happened to be passing at this moment, attended by two or three dogs, was able to put the whole matter in its proper perspective" (69-70).
His simplification of the changes advanced the ideas that the pigs needed their rest, required brain food, were the only ones capable of doing academic things, and held the cohesion of the farm sacred above all other things. None of these things were actually true. Instead, the pigs were merely making themselves more comfortable, staying better fed, pretending to make important decisions, and destroying the unity that once held them all together. And no matter what went wrong on the farm, Snowball was always to blame.
Squealer's persuasive approach was effective because of the way he delivered any news, skipping from side to side and whisking his tail as he did so. His movements held the animals' attention, while his entourage threatened them into believing him, and his perfectly-timed question, "Surely, you don't want Jones to come back?" scared them into fearing that if they didn't take him at his word, they'd be doomed.
Of course, in the end, they are doomed. The final scene illustrates the fact that despite the promises and propaganda spouted by Squealer, power has gone to the pigs' heads. They are indistinguishable from the humans, and ultimately the animals' fate is worse under their leadership than it had ever been under Jones's.