Civil wars are a feature in which a country, divided over ideological differences, splits into two warring states, each with the goal of the complete annexation of the other. As long as the conditions are met, any country in the game may experience a civil war.
Civil wars may occur based on the popular support for a certain ideology. If the support for a certain ideology that is not currently ruling in a country grows too large, they will start demanding a change of government. If the player ignores the clamor for reform the chance of a civil war breaking out will grow. It will then start with an event allowing the player to choose their side. It is also possible for the growing ideology to start a coup, which would allow for a peaceful transition.
A puppet cannot transition to another government peacefully; if the support of an ideology grows too high they will plunge into civil war which may result in the puppet breaking free if the rebellious ideology wins in the civil war.
Influencing popular opinion
A country may influence ideological makeup of it's own population by selecting the appropriate minister, this will result in an event in which they may choose between pushing internal support for a coup or popular support which will speed up the rate at which the ideology in question grows. Although choosing popular support does not make it impossible to have a coup committed, it does improve the chances of the civil war event firing before the coup. While a minister promoting a certain ideology is active occasional events may occur that influence the ideological make-up of the country or add National Spirits that will speed up ideological conversion.
Alternatively, a country may also influence the ideological makeup of a foreign country by supporting it at the cost of Political Power. Other countries may return the favor as well; this means civil wars may be actively pursued, but can happen even if not desired, if political support for a different ideology reaches high enough levels.
Once a Civil War breaks out, the country is divided into two. How it divides varies for non-scripted Civil Wars; the amount of land given to a side depends on the popular support for the ideology leading it. The two sides immediately enter a state of war with the opposing side, fighting to unite the Country under their flag. Once started, a civil war will act as a normal war with victory points eventually leading to capitulation as they fall. The only difference being that there are no occupation laws for occupied territory owned by the other side.
Pre-scripted civil wars
Historical civil wars usually occur when it did historically, currently the Spanish Civil War is the only example of this. In this case the division of the country is based on historical circumstances. Although pre-scripted, these civil wars may also be affected by foreign influence and support, volunteers and expeditionary forces, and may force other countries to intervene.