From Conflict of Nations Wiki
Insurgents are the result of rebellions within a nation. They act as a global threat and are listed as their own nation under the campaign's rankings. Insurgents can only spawn/uprise in cities. If you let these insurgents fester they will spread to the neighbouring provinces, keep this in mind when conquering cities as the morale will be low.
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.
- If you have units stationed in a city/province which is hit by an insurgent spawn, they will engage in combat. In that case, your units will benefit from the defensive garrison bonus.
- If a province/city's morale is low enough to be at risk of insurgency, it will be indicated through pillars of rising smoke.
Insurgents will gather over time, increasing in health points and stack number. If not stopped, they will begin to spread to surrounding provinces.