Warfare is the use of military force to engage in conflict with other nations, involving both land warfare, engaging other armies and sieging down cities and territories, as well as naval actions, fighting opposing navies in order to bloackade and assault ports and supporting land operations. The primary goal of a war is typically to enslave enemy pops by occupying their territories and accumulating enough warscore in order to press demands such as conquering territory and subjugating other countries.
Declaring war
War can be declared as the topmost option of the diplomacy interface, Declare War. After clicking the button, a wargoal with its associated casus belli can be selected. Any penalties will be listed on the panel to the left, and below the allies on each side and whether or not they will accept the call to arms.
There are several conditions under which a war cannot be declared, including if:
- The date is before 1 November 450 AUC
- The country is on the same side of another war as a target
- The country is
allied to, 
guaranteeing, or in 
a defensive league with the target - The target is a
subject of the current country - The country has at least
+100 opinion of the target - The country has
30 stability or less - The target is not in
diplomatic range
A few special events can also lead to declaring war, which can bypass some of these restrictions.
Casus Belli
- 主条目：Casus Belli
The aggressor may pick a casus belli and an associated wargoal when declaring war. The wargoal will determine the conditions for gaining ticking warscore modifies the warscore and aggressive expansion costs of each peace term term. If the wargoal is directed at a particular province that is later lost by outside the war, the wargoal will be removed on the war will be fought without a wargoal.
War declaration penalties
Under certain circumstances, a declaration of war will carry certain penalties, always in stability and sometimes in
war exhaustion and 
aggressive expansion as well.
No Casus Belli
Declaring war without a casus belli will cost the attacker a base of 25 stability, 2 war exhaustion, and 5 aggressive expansion, modified by the 
No casus belli war cost modifier. Only Superiority wargoals are available for wars declared without a casus belli, which triples the warscore cost of all demands.
Good Relations
If the target country has at least +50 opinion of the aggressor, the country declaring war will lose 10 stability upon declaring war.
Great Relations
If the target country has at least +100 opinion of the aggressor, the country declaring war will lose 20 stability upon declaring war.
Breaking a Truce
Attacking a country that the current country has a 
truce with will cost 50 stability, 1 war exhaustion, and 25 aggressive expansion. This penalty is also applied if declaring war countries that they have guaranteed, though not on allies or subjects.
Sides in a war
A war consists always consists of two opposing sides, each containing one or more countries.
War Leaders
While wars can have many participants, the country declaring the war and the country being declared upon will initially be considered the War Leaders. If the target country is a subject or is being guaranteed by another country, war leadership will automatically be transferred to the overlord or guarantor (if the call to arms is accepted), who will then be able to call in their own allies and subjects. In the case where one of the war leaders is annexed or otherwise destroyed while the war is ongoing, war leadership will be automatically transferred one of the secondary participants.
War Leaders will negotiate on behalf of the entire side (including all allies and subject countries) to which they belong, and a war will not end until the two War Leaders agree to a peace. Therefore when at war it is usually best to target the war leader's territories and armies in order to make them more amenable to a favourable peace.
Secondary Participants
Many diplomatic treaties are signed with the explicit intention of obligating states to help each other in wars. The call to arms usually happens immediately after the declaration of war, though if new treaties are signed early on in the war new countries can be called in while it is ongoing.
Even after signing the treaty, most countries can choose whether or not they want to accept the call to arms. Refusing the call will usually lead to the treaty being broken and a large loss of opinion from the country that made the call. A country will be more likely to accept a call to arms if:
- It has a high opinion of the country calling it
- It has a low opinion of the target
- It shares a border with the target
- The side it is being called into is stronger than the other side
A country will be less likely to accept a call to arms if:
- It has a low opinion of the country calling it
- It has a high opinion of the target
- Its
manpower is low - It does not share a border with the target
- It does not have any way for its military to get to the target
- The side it is being called into is weaker than the other side
Allies
Allies can be called into all wars that a country is the war leader of, and in particular is the only treaty that allows a country to call another independent country into an offensive war. If an ally has obligations towards both the attacking and the defending side of a war, it cannot be called in by the attacker and will always consider the defensive call to arms first, usually leading to the alliance breaking, unless the call to arms is refused.
Guarantors
If a country is guaranteed by another country, that guarantor will be called to arms and become the war leader if they accept. The initial target will still be able to call in their allies and defensive league members, which means that two sets of alliances may be involved in the defending side if this is the case.
Defensive Leagues
If a member of a defensive league is attacked, all members of the defensive league will be called to arms.
Subjects
Subject nations that can be called to arms are automatically called when their overlord enters the war, including as secondary participants. Unlike treaties with independent countries, whether or not subjects accept is dependent entirely on their 
loyalty; loyal subjects with at least 33 loyalty will always accept the call, while disloyal subjects under that threshold will never accept. This applies to human-controlled subject states as well.
If a subject nation is attacked, their overlord will always be automatically called in and become the warleader.
Warscore
- 主条目：Warscore
Warscore is a measure of how well the war is going for the country's side and ranges from -100 to 100. It is accumulated by winning battles, occupying enemy territories, blockading hostile ports, and fulfilling the wargoal, and is lost when the other side does the same. The warscore for one side is always the negation of the warscore for the other side, and it determines whether or not demands can be made of the other side and how willing they are to accept a certain peace offer. Warscore is calculated both for the war as a whole and for each individual country; the latter is used when negotiating a separate peace.
War Exhaustion
- 主条目：War exhaustion
War exhaustion represents the weariness and discontent that builds up from casualties and enemy actions during a war. It is increased by taking casualties and attrition, occupied territories, and enemy blockades, and will eventually tick inexorably upwards in a Call to Peace if a war drags on for too long. While War exhaustion increases 
military experience gain, it also significantly decreases happiness of all pops and impacts ruler popularity, which will destabilize the nation if not kept under control and is one of the more important factors in determining when it is better to sign a peace and leave the rest for now rather than try to press onwards in hopes of further gains.
Sue for Peace
The "Sue for Peace" screen can be accessed either by clicking on the war icon near the lower right corner of the map, or by opening the diplomacy screen and clicking the Sue For Peace button. Suing for Peace with the warleader of a faction will create a peace with all enemies in that alliance. Suing for Peace with an enemy that is not the warleader can conclude a separate peace with just that nation.
Peace conditions have a cost in warscore as well as possibly Aggressive Expansion (AE). The costs for the demands are listed beside them, and also totaled near the bottom right of the dialog box. Demands that total less than the accumulated warscore will likely be accepted by the enemy side. Demands that total more are likely to be rejected. In order to make any demands at all, a country must have at least 10 warscore, and the demands of a peace deal can never exceed 100 warscore.
War Enthusiasm
War enthusiasm is a measure of how willing a country is to continue the war, displayed under the name of the country in the main war interface with a detailed breakdown available in the tooltip. It is the main components in determining how likely a country is to accept a peace offer; a country will only accept a peace offer if the warscore of the country making the demand is greater than the sum of the target's war enthusiasm and the warscore cost of the peace treaty.
War enthusiasm is increased if:
- The war has not lasted for very long (less than 38 months)
- The country controls its capital
- The country won its last major battle
- The country's side has fulfilled the wargoal
- The country's side is stronger than the other side
- The country's side's warscore is increasing
War enthusiasm is decreased if:
- The war has lasted for a long time (more than 38 months)
- The country's side has not fulfilled the wargoal
- The country's side's warscore is decreasing
- The country's territory is occupied or under threat
Peace Terms
Cede Territory
The most basic peace term is to cede territory that the victorious side has occupied. Each individual territory will always be ceded to the country that currently controls it and can only be demanded if it is coastal and within the demanding country's 
control range, adjacent to a territory it already owns, or connected to another territory that fulfills one of those two conditions through other territories that are being demanded by that country in the peace deal. These restrictions can by bypassed, however, if one side is instead offering terms to other; any unoccupied territories that are offered up will be cede to the warleader if accepted. Each territory demanded will give the demanding country a base of 
1 aggressive expansion, adjusted by its 
aggressive expansion impact.
The warscore cost of each territory is much more variable and is affected by many different factors. It is increased by:
- Higher population
- Higher number of buildings
- Port status
- Belonging to a small country
- Having state culture and religion
- Not being the main target of the war
- Using the Superiority wargoal
Warscore cost is decreased by:
- The demanding country's
warscore cost modifier. Such as:
- Having adopted the
Bellicose diplomatic stance
- Having adopted the
- Having claims on the territory being ceded
- Using the Conquest/Claim war goal
- Belonging to a Regional, Major, or Great Power, if the demanding country has the same or lower rank
Each territory always costs a minimum of 1 warscore and the final cost is always rounded up, to a maximum of 99 warscore.
Cede Province
File:Cede state.png Rather than deal with individual territories, it is also possible and more common to demand that the opposing side cede provinces instead. The side that is demanding a province must control the provincial capital and any forts in the province, and will be transferred to the controller of the provincial capital if accepted (even if other territories in the province are controlled by other participants in the war). A province can only be demanded if all of its territories are accessible (it is coastal and within the demanding country's control range, adjacent to a territory it already owns, or connected to another territory that fulfills one of those two conditions through other territories that are being demanded by that country in the peace deal) and like with ceding territories all occupation and access restrictions are bypassed if the province is offered up by the losing side instead. The aggressive expansion cost and 
warscore cost of ceding provinces is the sum of the cost of each individual territory, though the as the rounding and casus belli modifiers are not applied until after summing up the cost of each territory, it almost always costs less to demand the entire province in a single peace deal term than it does to demand each territory separately. The cost of demanding a province is capped at 99 warscore, even if the sum of the warscore cost of the individual territories is greater.
Release Nation
A country can also demand that its opponents release nations whose former territories they control in order to weaken them. A country can be released if it no longer exists on the map and the losing side owns territories that they once owned and still contain pops of their primary culture; however, the exact territory that is given to the released country may differ from the territory that they last controlled and depends on the cultural makeup of the area as well as the extent of their claims, with the peace term often preferring to release whole states rather than splitting them up. A country cannot be released if any of the territory that it would released from is involved in another peace term, such as being ceded to one of the war participants or being released as part of another released country's territory. As well, only the country that is currently being negotiated with can be forced to release nations, so a separate peace is required to force secondary participants to release territory. Unlike most other peace terms, releasing nations does not require that the winning side control any territory in the area and does not cost any aggressive expansion; the warscore cost is half of the total cost of the territories that will be released.
Cancel Subject
The victor of a war can also demand that the defeated countries to cancel subjects that they currently have, releasing them as independent countries. Any subject can be released, even if they are unoccupied or not involved in the current war at all, but as with releasing nations only the country being negotiated with can be forced to release subjects; a separate peace is required to force secondary participants to cancel subjects. Cancelling subjects does not cost any aggressive expansion and as with releasing nations the warscore cost is half of the total cost of the territories of the subject that will be released.
Become Subject
Rather than directly annexing territories from the other side, a country can demand that an defeated nation become a subject. Any country on the opposing side can be forced to become a subject if their capital is occupied, including secondary participants and states that are currently subjects or other countries involved in the war, but only one country can be vassalized for each peace deal (requiring separate peaces to vassalize multiple countries in a single war), and only if none of their territory is currently being annexed. A country can be forced to become any subject type whose requirements it and the future overlord meet, with all possible types costing the same amount of aggressive expansion and warscore. Forcing vassalization costs half of the total aggressive expansion and warscore cost that annexing the entire country would.
Separate Peace
If a country has been completely occupied or 36 months have passed since the war began, the war leader and any independent, non-warleader participant on an opposing side can negotiate a separate peace, where peace terms are decided only between the war war leader's side and the individual participant who will leave the war after the terms are agreed upon. Two secondary participants on opposing sides cannot sign a peace with each other. Warscore in a separate peace is determined only by occupation; warscore from battles and the wargoal are not applied. As the maximum peace treaty warscore cost is applied to each separate peace independently, it is possible to take far more territory in a single war by negotiating with each opponent separately than would be possible if negotiating only with the opposing war leader. There are no extra penalties to aggressive expansion or warscore cost in a separate peace, though the warscore cost increase from making demands of junior war participants remains.
Truces
After a peace has been signed, a truce will be in place for between 3 and 8 years, scaling linearly based on the warscore of the peace agreement (0.6 months per warscore of the peace treaty, rounded to the nearest month), between all participants of the treaty. Breaking the truce by declaring an offensive war will cost 50 stability, 1 war exhaustion, and 25 aggressive expansion. Attacking countries guaranteed by a state you have a truce with is considered breaking the truce and will result in the same penalties, but truces can be circumvented by attacking their allies or subjects (which is typically limited to tributaries, as allies and most other subject types are called in during a war).
Revanchism
The losing side of a war will gain revanchism if it loses territory in the peace treaty, allowing defeated nations, particularly larger nations that still have a significant amount of territory after a defeat, to bounce back more quickly rather than fall into an irreversible decline as other countries take advantage of its weakened state and it cannot muster enough 
or to defend itself. A nation will gain 1 point of revanchism for each point of warscore in territory that is taken in a peace treaty, capping at 100. Revanchism stacks across successive peace treaties, but declines by 0.833 each month. Each point of revanchism has the following effects:
+0.5% National Tax
+0.15% Fort Defense
+0.25% Manpower Recovery Speed
-0.05 National Unrest
+0.0001 Monthly Legitimacy
+0.20% Pop Promotion Speed
