Why do the pigs change the truth about Snowball & blame him for the problems of Animal Farm?
On one level it is important to understand that Animal Farm is an allegory of the Russian Revolution, and that Snowball is meant to represent Leon Trotsky, the Bolshevik hero of the Revolution and the Civil War that was driven into exile and murdered by Stalin, who perceived him as a rival. On another level, Orwell uses Snowball to show how totalitarian governments use fear to govern. It is not enough that the animals fear Napoleon. In order to maintain power, Napoleon must create the illusion that there is a threat to the farm that only he is strong enough to resist, and that justifies extreme measures. Thus the pigs use Snowball as a scapegoat for everything that goes wrong on the farm, and portray him as a looming, sinister presence, always ready to destroy the society the animals have created on Animal Farm. When Napoleon orchestrates his bloody purges, the crime his victims are accused of is colluding with Snowball. Snowball is also used to show how totalitarian propagandists can manipulate the past. Squealer, the lead propagandist for the pigs, attempts to downplay Snowball's military heroism after his departure, say of his valor at the Battle of the Cowshed:
I believe the time will come when we shall find that Snowball’s part in it was much exaggerated.
Indeed, it is not long before the animals are told, and mostly believe, that Snowball actually led the enemy at the Battle of the Cowshed. The recreation of Snowball after his exile is an example of a maxim from Orwell's other great work, 1984, that "who controls the past, controls the future."