The armies they get and manage to field will tend to remain within the territory of their respective settlement and focus on defending from attacks rather than trying to expand their land. Any settlements they do take will only join them from a revolt, not by conquest. Thankfully, the Rebels seem to be more passive than offensive. It's also likely that the Rebels will have a better economy than the Western Empire at the start of the game, allowing them to upkeep their new armies and settlements. Not only that, but they will often take away some of the units already in that settlement when they revolt as well as generating new units based on the buildings currently built in the settlement. They not only take away both units and settlements from the Western Empire, but they are also immediately at war with them.
If playing as the Western Roman Empire, the Rebels will prove a big problem. Their starting armies are also random, based on the units in those settlements that revolt which decide to support the rebels. Since they are an emergent faction, the date that they appear on and what settlements they start with are also completely random based on the unrest level of settlements in the Western Roman Empire.
As such, the Western Rebels have no victory conditions and not much aim other than surviving the campaign to the end. The Western Roman Rebels are not playable in the campaign without using mods.
