From Conflict of Nations Wiki
Insurgents are the result of rebellions within a nation. These act as a global threat and are listed as their own nation under the campaign’s ranking. Insurgents can only cause an uprising in cities that have a low morale but if left to fester they will spread to neighboring provinces, conquering and expanding. When conquering a city, morale will be low, be sure to monitor your Target Morale and leave a garrison while the city recovers. Otherwise, the city may rebel and spawn insurgents which can cause you problems later on.
How Insurgents Spawn
Cities and are at risk of rebellion while their morale is below ~35%, but this risk increases greatly when the morale of a city is under ~25%.
- Undefended cities and provinces are immediately captured the moment insurgents spawn on them, but insurgents will not conquer the city or province until your troops are defeated
- If you have units stationed in a city/province which is hit by an insurgent rebellion, they will engage in combat. In that case, your units will benefit from a defensive garrison bonus.
Insurgents will build up over time, swelling in numbers and spawning more insurgent units. If they aren’t dealt with sooner rather than later they will begin to spread to surrounding provinces. It is possible for insurgents to eventually reach other cities, conquering them, and begin to spawn insurgents from the newly conquered city as well. This can cause a cascading effect that you should be careful of.