A rebellion is a kind of built-in event which causes a rebel army to pop into existence on the map. A rebellion is distinct from a revolt (a country declares independence), although many players use the terms interchangeably.
Effects of Rebellion
A rebellion has one typical effect: a rebel army is created in a settlement. The size of the army is determined based on the settlement's population and fortress level. If the rebels capture the fort (see below), or the province is unfortified, then the rebel army will typically only be a few thousand men, or even fewer for colonies. If there's a fort and the rebels don't capture it, then the rebel army will always be large enough to siege the fortress: it will have 6000 men per level of the fortress.
There is a second effect of rebellion that happens, although rarely. Occasionally rebels capture the fortress without a siege. (In an unfortified settlement, the settlement is always captured immediately unless there is a friendly army present.) In a fortified city, there is a chance, increasing with net revolt risk but decreasing with the fortress level, that the fortress is captured. For the typical ranges of net revolt risk that most players will accept, the chance of fortress capture is very low. However it appears to max out at 42% or so for very high levels of revolt risk.
For more on this, see this discussion at the forums. More EU2ology needs to be done for us to know the exact details, but the general picture is clear: keep down revolt risk and captures rarely happen.
Causes of Rebellions
Events can trigger rebellions in specific provinces, or random ones. Many countries have country-specific scripted events containing rebellions, and there are also random events with them. (Random events apply to all countries.)
Outside of events, rebellions happen for two reasons. One is, the owner has tried a religious conversion in a settlement and the conversion completed, but failed. This automatically causes a rebellion. The other is because there is nonzero net revolt risk in a city and it has failed a rebellion check.
A rebellion check is a game engine subroutine where every city controlled (not owned) by a country is checked for a rebellion, using the city's current net revolt risk. Rebellion checks happen in the following situations:
- On the first of every month, to all countries.
- Each time a human player closes his religion screen, if he has changed any religious tolerance slider.
- Each time a country has a stability hit if it is already at minimum stability. For example, if you break a truce (-5 stability) when at -2 stability, you'll face four rebellion checks.
The chance that a rebellion check causes a rebellion is the displayed net revolt risk in the city, divided by 12. For example, if a city shows 6% revolt risk, its chance of a rebellion is 0.5% on each rebellion check. Thus, the net revolt risk is (roughly) a per-year figure for the chance of a rebellion.