Civil wars are a feature in which a country, divided over ideological differences, breaks into two separate 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 with any give ideologies pitted against each other.
Starting the 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 or actively refuses to allow 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.
Influencing popular opinion
A country may influence ideological makeup of its 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 another country by supporting it at the cost of Political Power. Other countries can do the same thing 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. Certain National Spirits and government ministers apply a percentage-based "Ideology Drift defense" stat to their country. This stat reduces the efficiency of ideology-boosting actions, which can reduce the chances of a civil war breaking out due to foreign ideology support.
Civil wars can also be triggered by foreign-funded attempted coups. For this, a country must supply 200 Political Power (0.5 per day) and Infantry Equipment. The amount of infantry equipment required appears to be a function of the target country's civilian population. The more populous the country, the more infantry equipment a nation has to give in preparation for the civil war.
Once a civil war breaks out, the country breaks into two separate entities. 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. For this reason, for effective coups, it is recommended that the "Stage a coup" action is coupled with the "Boost party popularity", as increasing the support for an ideology higher will also increase the size of the break-away state. The two sides, the loyalists and the rebels, immediately enter a state of war with each other, 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.
Splitting the country
When the civil war starts, the country is split in two, in proportion either according to a pre-scripted event, or to relative support of warring factions. For example, the pre-scripted Spanish Civil War is a 50%/50% split, the pre-scripted French Civil War is an 80%/20% split and the pre-scripted Soviet Civil War can be a 82%/18% split, a 70%/30% split or a 50%/50% split depending on the circumstances.
Divisions, ships, air forces, stockpiled equipment and divisions in training are split according to the percentage.
Both countries inherit all completed and in progress focuses and research.
The countries are not created equal though. One of them is considered the original, and another one the splinter nation. The original nation keeps its country tag even if it now has a different ideology and name, whereas the splinter nation gets a new tag.
The original nation gets to keep all hired political advisors, research and military staff, as well as generals and admirals. It also keeps all accumulated political power, army/navy/air xp, and unspent research bonuses. The splinter faction gets none of that.
Influencing ongoing civil wars
Civil wars can be influenced by foreign nations in many ways. The two most notable include sending volunteers and lend-leasing equipment. Once a civil war breaks out, the nation that supplied Infantry equipment for the rebels can offer further support by providing additional equipment through Lend-Lease. In addition to supplying basic things such like Infantry Equipment, a nation can also choose to provide advanced equipment like Tanks and/or Artillery for the rebels. This can allow the rebels to more effectively supply their military which can grant them an edge over their opponents.
Sending volunteers is another way to influence the progression of a civil war and can also be used to gain combat experience without actively joining the war. In order for a nation to be able to send volunteers to aid one side in a civil war, the sending nation must be at peace with the target side (though not necessarily the opposing side) and must not be in a faction with the side they intend to support. In addition to that, the sending nation must have enough army divisions deployed to send volunteers.
The number of divisions that can be deployed is 1 division for every 20 active divisions (rounded down, so with 19 or fewer no Volunteers can be sent). For example 176 active divisions means 8 divisions where 181 would mean 9 divisions could be deployed as volunteers. Note that a division with only 1 battalion in it counts as much as one that has 25. The minimum of 20 divisions with 1 infantry will enable 1 volunteer division to be sent. The sending country must have at least 30 divisions to send volunteers at all. Added to this is one division per 20 provinces in the destination country.
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.
Civil wars can be incorporated into military strategy if one so chooses. A nation can choose to start a civil war in a another nation to weaken them prior to invading them. An example could be Germany starting a civil war in the Soviet Union to weaken the country before invading it.
However, one should be mindful that if a break-away state is fighting a civil war and its opponents are taken out by a foreign power, the break-away state will retain control of its initial territory while the foreign powers will only be able to make demands on the nation they targeted, due to them being separate entities. This means you will not be able to make any demands on territory controlled by the rebels once the peace conference starts, meaning that starting a civil war to safeguard an invasion turns the conquest into a two-stage affair. Invading and defeating the target nation first, then invading and defeating the rebels after that.