After Snowball's expulsion, Napoleon slowly begins to take greater and greater control of the farm, partly through military coercion (with his trained dogs) and partly through propaganda techniques. Although the reader understands how he is seeking power, the animals are slowly moved into the position of slaves, and so do not notice Napoleon's changes until it is too late. His most important change, the one that directly contravenes everything that Old Major's philosophies stood for, comes as the farm becomes dependent on trade instead of being self-sufficient:
Nevertheless, as the summer wore on, various unforeseen shortages began to make them selves felt.
...Napoleon announced that he had decided upon a new policy. From now onwards Animal Farm would engage in trade with the neighbouring farms: not, of course, for any commercial purpose, but simply in order to obtain certain materials which were urgently necessary.
(Orwell, Animal Farm, msxnet.org)
This new dependence is hidden behind the pretext of using trade to fortify the farm's position, but in fact it shows a more important message; when the equality of labor on the farm begins to shift, and some animals start consuming more than they produce, the entire economy breaks down. Trade with humans violates Animalism, but is excused as necessary for the survival of the farm. The animals forget that only a year earlier, they were entirely self-sufficient, and the division of labor was much more equal.