Insurgencies

General

Insurgents are the result of rebellions within a nation, but act as a global threat.

How Insurgents Spawn

This only applies if the morale is below ~40% If the insurgents spawn, and a unit is not found within that city, then the insurgents will take over the city and slowly garrison.
If you have units allocated within that city, your unit will fight them and will also have a garrison bonus which will help them. So prevention is better than cure as you will have the upper hand.

Garrisons

Insurgents will gather over time, increasing in HP & stack number, as well as spreading to surrounding provinces.