why does Napoleon revive the threat of the farm being sabotaged by Snowball?

If Napoleon can convince the animals that they are in danger of attack, he has reason to develop his attack dogs and to make the animals ready to fight when Snowball might attack. He cements his power in this way.

Napoleon wants to unite the animals against a common cause. Using a threat of war from a neighbouring farm makes the animals frightened and band together, making them less likely to rebel within Animal Farm.