Rebels are a gameplay mechanic in Age of Civilizations 2.
Rebels appear when a province has a high revolt risk. This revolt risk increases when the population in the province is unhappy, which can happen when the AI decides to plunder a province. after some amount of turns, the rebels will declare independence and all the land that they hold at that moment will change ownership. A new country will be created, with its own flag.
Rebels usually have a total army size of a thousand, or maybe 5000, but these armies are spread all over the provinces.