It's not in Squealer's interest to tell the truth about the pivotal role played by Snowball during the epic Battle of the Cowshed. He knows as well as anyone that Snowball completely outshone Napoleon, who went nuts while the battle was raging. So once Snowball has been driven away from the farm by Napoleon's attack dogs, Squealer sets about systematically trashing his reputation. No longer is Snowball the hero of the Battle of the Cowshed; now he's nothing more than a traitor and a criminal.
Squealer drives this point home by gaslighting the farm animals. They clearly remember the key role that Snowball played in the great victory over the hated human oppressor. But Squealer persuades them that their memories are somehow faulty—that what they think they saw never really happened. What Squealer's doing here is constructing an alternative version of history in which Napoleon was the real hero of the hour while Snowball was just a sniveling coward who ran away from the heat of battle.
This is a classic example of thought control, a common tactic among totalitarian regimes. In this role as chief propagandist of Napoleon's regime, Squealer uses his skills as an orator to manipulate the animals into believing a completely false, distorted version of history.