While Animal Farm was written as a fable utilizing the Russian Revolution and the years that followed, the story has an even greater implication. It shows how a dictator is gradually able to take over the minds and souls of his or her people. Note that after the revolution on the farm, all animals enjoy a hint of equality for a very short time; then, very shortly, milk and apples become luxuries reserved for the pigs alone.
In time, history is re-written and the great things that Snowball accomplished are taken away from his credit. In fact, Snowball becomes a villain almost on the same level as Jones himself.
The old saying that "Power corrupts and absolute power corrupts absolutely" are among the many ideas that Orwell communicates.