One night after Jones went to sleep Old Major gathered everyone in the barn. He gave a speech of inspiration on a rebellion and expelling Jones. The condition Jones kept the animals in was enough for them to rebel against him. Though Old Major died two pigs, Snowball and Napolean, took charge of the rebellion.
After the two were in power it was almost impossible for them to keep the peace. They argued over the smallest things which eventually lead to bigger things. One of them would eventually gain more power and overthrow the other. In this case Napolean overthrew Snowball.
7 Commandments: 1. Whatever goes up on two legs is better 2. Whatever goes upon four legs, or has wings, is inferior 3. No animal shall wear clothes except pigs 4. No animal shall sleep in a bed with sheet 5. No animal shall drink alcohol in excess 6. No animal shall kill another animal without cause 7. All animals are equal some more equal than others
When Napolean starts to gain power everything goes downhill. The regular Sunday meetings were closed. Instead, Napolean held meetings with exclusively pigs. This is already breaking one of the seven commandments. This will soon lead to the downfall of the farm.
Finally, Napolean managed to kick Snowball off Animal Farm and gain complete power. After Snowball's expulsion, it was easy for Napolean to push the issues of the farm onto someone else. That someone was Snowball. Since Napolean portrayed Snowball to be a "traitor" it was quite easy for the animals to believe that Snowball was the root of their problems.
You will pay for this!
Power changes man. This was the case with Napolean. As Napolean grew more and more in control of everything that went on in the farm he changed the commandments to his liking. the animals were too "dumb" anyways so why not add a couple of words here and there. That was how Napolean was thinking. Little did the animals know that being ruled by their own kind was worse than being ruled by man.
Towards the end, the pigs are in alliance with the humans. Despite them telling the animals of the evil the humans have done, they infringed upon their own rulings. Not just four legs good two legs bad, but every other commandment. at the very end of the story the animals, dare they think, that maybe Jones was better than the pigs. There was no difference between man and pig or pig and man.