Before the rule of the princesses, ponies were split in different factions: Earthponies, Unicorns and Pegasi.
Due to certain circumstances, these groups unified under one banner, starting to cooperate in friendly manner instead of draining each others needs.
But revolutions are never friendly, and there are always oppositions.
Commander Lance Skydiver from the Pegasus's fortress in the north of Equestria is a purist: Pegasi are superior, and should rule over the others. When the unification started, he knew that he had to act quickly. He gathered a thousand fellow Pegasi Purists and went to the north. At first, everything seemed fine, but the united forces approached from south, and the siege of the fortress began.