In the first chapter of Animal Farm, Old Major is inspired to give a speech to the other animals after having a strange dream. While he does not reveal specific details, he says that the dream represents "the earth as it will be when Man has vanished." Through his rousing speech and his teaching of the song, The Beasts of England, Old Major awakens the animals to the idea of revolution.
Major does not suggest to the animals that the pigs should lead this revolution but after his death, at the beginning of Chapter Two, the pigs are generally accepted as the leaders. There are two reasons for this development: firstly, that Major's message appears to affect them the most, and secondly, that they are "generally recognised as being the cleverest of the animals." The pigs immediately set about preparing for the revolution and develop a "complete system of thought" which they call Animalism. This provides the ideological basis for the revolution which then takes place.
Being versed in ideology, then, the pigs are able to answer questions put to them by the other animals on the forthcoming revolution. When Mollie asks, for example, if there will be sugar after the revolution, Snowball is able to answer her question. While Mollie does not agree with Snowball's assertion that she will not need sugar, she cannot deny the confidence and intellectual superiority of the pigs and this justifies their leadership in the rebellion.
But there is a problem with the pigs' leadership. Power soon begins to corrupt many of their number. In fact, Snowball is the only pig who is not corrupted by his position but he is driven off the farm by Napoleon, in a display of his tyrannical power.