Revolutions fail because leaders pursue their own interest as oppose to the interest of those whom the revolution was meant to serve. Critically...

By the end of the book, the revolution that established Animal Farm has taken exactly this course. The pigs, who emerged as leaders of the uprising and in implementing Animalism on the farm, quickly become corrupted with their newfound power. Even before Snowball is purged from the farm, the pigs are allowing themselves extra privileges, like milk and apples in their mash. Over time, Napoleon continues to abuse the considerable power he has amassed. He trains fierce attack dogs that are used to kill those whom he views as disloyal. He and the other pigs begin to drink alcohol, sleep in the farmhouse, and even wear human clothes. They engage in shady deals with humans outside the farm, some of which go very badly. Squealer justifies their behavior to the other animals, and the pigs gradually alter the Seven Commandments to read that some animals are more equal than others. In the end, the animals are more miserable than they were under Jones, even loyal Boxer has been sent to the knacker (just as Old Major, ironically, said he would be by Jones) and the pigs live in luxury in the farmhouse, gambling, drinking, and consorting with humans. By the end of the book, the revolution has thus failed to achieve the aims that Old Major laid out in the first chapter. The pigs are indistinguishable from the humans, who they have matched in their greed and cruelty.