Orwell's story is illustrating the fact that revolutions often result in worse conditions for the people involved than they suffered under before. This was the case with the Russian Revolution, which is what Animal Farm is referring to allegorically throughout. After the Czar was overthrown, the Russian people expected freedom, equality, and prosperity. Instead, a dictatorship comparable to the pigs in the novel took over the government and imposed force and terror on the people. Millions of peasants died of starvation because they were opposed to collectivization. Millions were sent to concentration camps. The oppression was begun under Lenin but became even worse under Stalin. Orwell was a socialist himself, but he believed in democratic socialism, wherein the people have the final authority, as in Britain and the United States. Revolutions usually result in dictatorships under which the people are oppressed almost as severely as they were before. The major problem, as Orwell seemed to view it, is with human nature. People are selfish, and the most selfish tend to rise to the top. Napoleon in Animal Farm is a good example. The honest, hard-working people will always be exploited, like Boxer.
The serious effect of propaganda on feeble minds, especially on "pure" people that accept anything given by an authority as something worth believing
Orwell is trying to state (using the Russian Revolution as an example) that power corrupts and absolute power corrupts absolutely. Prior to the animals rebelling against Mr. Jones, Major gathers the animals together in the barn (his character represented a mixture of Marx and Lenin.) He speaks on how the farm can be changed for the better. The revolution comes after Major has died and Snowball (representing Trotsky) and Napoleon (representing Stalin) take over with Squealer as a speaker (who represents the propaganda throughout the events.) The pigs start out with the best interests of all the animals in mind but then, beginning with the milk and apples, they succumb to their power. When Jessie and Bluebell give birth, their puppies are taken for "special education" (they are raised to be the book's parallel to the KGB.) As the pigs continue using and abusing their power, the conditions for the animals get worse and worse while they are under the impression that they have actually gotten better. In the end, you see that Napoleon has been completely corrupted and is no better, if not worse, than the humans on the neighboring farms.