Snowball's expulsion is the major turning point, as this gives Napoleon freedom to institute his decisions without debate. However, the corruption of power in the farm was seen much earlier, when the pigs decided that they should receive a better portion of food than the other animals. Setting the milk and windfall apples aside for the pigs is the first overt clue that the balance of power and privilege is not as equal as Old Major had wanted:
"Milk and apples (this has been proved by Science, comrades) contain substances absolutely necessary to the well-being of a pig... The whole management and organisation of this farm depend on us... Do you know what would happen if we pigs failed in our duty? Jones would come back!"
(Orwell, Animal Farm, msxnet.org)
Using fear and circular logic to get more food than the other animals is a sign of dishonesty; the pigs do not need any more or better food to perform their organizing tasks. However, being somewhat more intelligent, the pigs realized that they can take advantage of the "equal sharing" philosophy and excuse doing less work for more benefit. While Snowball was an idealist, truly believing that he could make life on the farm better through his own hard work, Napoleon was a cynic, working entirely for his own gain. Power on the farm thus slowly shifted from the masses to the pigs alone, with Snowball not quite understanding until it was too late. Napoleon's attack on Snowball was the inevitable end result of his need for power over equality.