Although Animal Farm is based on The Russian Revolution and is generally accepted as an allegory of revolution in general, the foundation of the problem is greed and abuse.
The animals overthrow their humans because they are being neglected and abused. If they had not been kept from food, they might never have rebelled even with Old Major’s dream. It takes a serious stimulus to be the impetus of action.
In past years Mr. Jones, although a hard master, had been a capable farmer, but of late he had fallen on evil days. (ch 1)
It is this abuse that causes Old Major to describe a dream where animals are treated well, and get to keep the products of their labors—since man is the only animal that consumes without producing.
Greed does not end with Jones, however. The revolution starts off well enough, with the pigs in charge. However, they begin eking out more comforts for themselves.
The animals had assumed as a matter of course that these would be shared out equally; one day, however, the order went forth that all the windfalls were to be collected and brought to the harness-room for the use of the pigs. (ch 3)
Equality does not last very long. As time goes on, the New Order, represented by the pigs, becomes more and more greedy. This is reflected in the changing of the commandments as the pigs sleep in the house, use human beds, eat milk and apples, drink alcohol, and wear clothes. Soon it is impossible to tell the pigs from the humans.