To keep power, Napoleon uses military might in the form of trained dogs; most of the other animals are prey instead of predator, and cannot defend themselves against nine large and vicious dogs. Aside from their first appearance, when they try to kill Snowball, this is seen in Napoleon's reaction to the rebellion of the hens; their eggs are being taken in large numbers with no regard for the need of raising chicks. The hens try to rebel by laying their eggs in the rafters; Napoleon and his dogs force the other animals to refuse them food until they are starved out. Later, Napoleon and Squealer become aware of the growing discontent in the animals, and take drastic action:
They were the same four pigs as had protested when Napoleon abolished the Sunday Meetings. Without any further prompting they confessed that they had been secretly in touch with Snowball ever since his expulsion... When they had finished their confession, the dogs promptly tore their throats out, and in a terrible voice Napoleon demanded whether any other animal had anything to confess.
(Orwell, Animal Farm, msxnet.org)
This is the first of the forced confessions, where animals create lies about their actions and are killed as a result. Some thought that they could avoid death by "confessing" to fake crimes, but Napoleon knows that without total control of the farm, he will lose his power. The dogs are Napoleon's army, and some of the animals act as "secret police," informing on the activities of others even when they are innocent. In this way, Napoleon cements his authority and make the other animals scared to think of rebellion.