Casus belli

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This article is accurate for the latest versions of EU3, Napoleon’s Ambition, In Nomine, Heir to the Throne and Divine Wind.

Casus belli comes from Latin, meaning justification for war. In the game, a country with a casus belli (commonly known as just "CB") on another country is deemed to have a valid cause of war. When declaring war, the player must choose a CB. A proper CB will result in less stability loss and less infamy accumulation compared to the same situation without a CB.

Heir to the Throne

The casus belli system has been completely revamped in the Heir to the Throne expansion pack (HTTT). Instead of just one type of CB, there are several from which the aggressor must choose at the onset of the war. Each CB becomes available in its own way, and each has its own expected War Aims.

No Casus Belli

Declaring war without a casus belli costs -2 stability and gives an immediate +2 Infamy.

Name of War: War of $ATTACKER$ Aggression

Casus belli and alliances

A casus belli only reduces infamy and warscore costs when negotiating with the original target of the war, not with other nations that may join the conflict. For example, if A declared Reconquest on B, and C joined B in defense, A could not take territory at a reduced cost while negotiating with C, even if C owned provinces that were cores of A and was thus itself eligible for the Reconquest casus belli.

For this reason, it is often advisable to simultaneously declare war against multiple countries. In the above example, if A declared separate wars of Reconquest on B and C, each country would be the original target of a casus belli and could thus have taken territory from it at no cost.

Mutual casus belli and defenders

Certain casus belli are mutual, allowing a victorious defender to benefit from the cost reductions of the casus belli. These benefits only apply to eligible provinces. For example, the Reconquest casus belli is mutual, but if A used Reconquest on B, B could only take infamy-free provinces from A if they were cores of B - any other territory would come at full cost.

If the casus belli is not mutual, defenders may take territory at 75% of base cost.

War Aims

Different CBs are designed with different war aims in mind. For example, some CBs may guide the player towards religious conversion or financial reparations. Others, such as the dreaded Excommunication CB, stop nothing short of the total eradication of the target state. While war aims do not by themselves restrict an aggressor's actions in a war, they have a significant impact on the consequences of the war: the eventual peace deal, infamy, and prestige.

Triggered Casus Belli

Some CBs are created by an event, whether by mission or as a consequence of your own actions. These have a duration measured in months.

Casus belli Negotiation modifiers
Name Months Prerequisites/Trigger Infamy Prestige Warscore Affected peace options Name of War
Defected province 12 A defected province 0% 200% 50% Demand provinces (only recently defected) $ATTACKER$ Reconquest of $PROVINCE$
Canceled loan 12 A canceled loan 0% 200% 50% Indemnities, Concede Defeat $ATTACKER$-$DEFENDER$ War of Recompense
Spy discovered 12 Discovered spy 0% 200% 50% Revoke cores, Indemnities, Concede Defeat $ATTACKER$ War over $DEFENDER$ Provocation
Disloyal Vassal 120 Vassal breaks vassalization 0% 200% 50% Become vassal $ATTACKER$-$DEFENDER$ Vassalization War;
Defender of the Faith 12 Be Defender of the Faith, Heretic/Heathen state attacks State Religion state 0% 200% 50% Release vassals, Release annexed, Revoke cores, Reduce sphere, Indemnities, Concede defeat $ATTACKER_RELIGION$ Defender War
Imperial Liberation 24 Be Emperor, target owns non-cored HRE provinces 25% 200% 50% Release annexed, Release vassals, Reduce sphere, Demand provinces (only uncored HRE provinces), Indemnities, Concede defeat Holy Roman Intervention
Guaranteeing Independence 12 Violated guarantee 0% 200% 50% Release vassals, Release annexed, Reduce sphere, Revoke cores, Indemnities, Concede defeat $ATTACKER$ Intervention
Warning 12 Violated warning 0% 200% 50% Release vassals, Release annexed, Revoke cores, Reduce sphere, Indemnities, Concede defeat $ATTACKER$ Intervention
Trade League War 12 Be Merchant republic, league member leaves 0% 200% 50% Join trade league, Indemnities, Concede Defeat $ATTACKER$-$DEFENDER$ Trade War
War of Honor 12 Insulted 0% 200% 50% Revoke cores, Indemnities, Concede defeat $ATTACKER$-$DEFENDER$ War of Honour
Dishonored alliance call 36 Alliance member dishonored call to arms 0% 200% 50% Revoke cores, Indemnities, Concede defeat $ATTACKER$-$DEFENDER$ War of Honour
Restoration of Personal Union 120 Broken personal union 0% 100% 100% Form personal union $ATTACKER$-$DEFENDER$ Unification War
Sphere of Influence 24 Sphere of influence violated 0% 200% 50% Annul treaties, Reduce sphere, Indemnities, Concede defeat $ATTACKER$ Intervention
Subjugation War 120 Vassalization mission 0% 200% 50% Become vassal $ATTACKER$ Subjugation of $DEFENDER_NAME$
Religious Nonconformist 24 HR Emperor, HRE member changes religion 0% 200% 50% Change religion Holy Roman-$DEFENDER$ War of Conformance
Border friction 12 Foreign national focus borders you 50% 100% 50% Demand provinces (only ones affected by national focus), Concede defeat $ATTACKER$-$DEFENDER$ Border War
Obscure Documents 12 Successful fabrication of claim on target's throne 50% 200% 150% Form personal union
Japanese Civil War DW.png 12 Claim title of kampaku 0% 150% 75% Endorse for shogun, stop endorsing for shogun, become kampaku Japanese Civil War

Automatic Casus Belli

Other CBs are created automatically and remain until a certain condition is fulfilled. Peace options generally apply only to certain goals, like a temporary claim or an unowned core province.

Casus belli Negotiation modifiers
Name Prerequisites/Trigger Infamy Prestige Warscore Affected peace options Name of War
War of Conquest Temporary claim granted by missions 25% 200% 50% Annexation, Demand provinces (only mission targets) $ATTACKER$ Conquest of $PROVINCE$
War of Reconquest Core province owned by foreign power 0% 200% 50% Annexation, Demand provinces (only cores) $ATTACKER$ Reconquest of $PROVINCE$
Call to Arms (also called "Alliance") Ally under attack with state with which you are at peace and have no truce 0% 200% 50% Release vassals, Release annexed, Reduce sphere, Revoke cores, Indemnities, Concede defeat $ATTACKER$ Intervention
Nationalism Government 30, Cultural union state, target owns provinces of your cultural group 50% 200% 50% Annexation, Demand provinces (only in culture group) $ATTACKER$-$DEFENDER$ Nationalist War
Imperialism Revolutionary Empire, Revolutionary Republic, Enlightened Despotism, Republican Dictatorship, Absolute Monarchy 50% 100% 75% Annexation, Demand provinces (any) $ATTACKER$-$DEFENDER$ Imperialist War
Revolutionary War Revolution and Counterrevolution NI and one of these must be true: republic government and monarchic target, monarchial government and republican target 25% 100% 50% Annexation, Demand provinces (any) $ATTACKER$-$DEFENDER$ Revolutionary War
Colonial War Quest for the New World NI, Colony Claim, Overseas Provinces 25% 100% 50% Demand provinces (only neighboring overseas) $ATTACKER$-$DEFENDER$ Colonial War
Liberation War Bill of Rights NI, Target can release vassals 0% 300% 50% Release vassals, Release annexed, Reduce sphere $ATTACKER$-$DEFENDER$ Liberation War
Holy War Before 1650, Christian or Muslim, target is of different religious group and one of these must be true: Be an Empire, Defender of the Faith, border target, be Catholic and target has Crusade called against it, Unam Sanctum NI, Divine Supremacy NI. 25% 150% 50% Annexation, Demand provinces (any) $ATTACKER$ Crusade/Jihad against $DEFENDER_NAME$
Religious Liberation Not Christian or Muslim, target is of different religious group, target owns a province from your religion group and one of the following must be true: Be an Empire, Defender of the Faith, border target, Unam Sanctum NI, Divine Supremacy NI 25% 125% 100% Annexation, demand provinces (same religious group) $ATTACKER$-$DEFFENDER$ Religious War
Purging of Heresy Unam Sanctum NI, Must Be In Same Religious Group, and one of these must be true: Have the "Heretic Country" Flag, Target have "Heretic Country" Flag, Target have different religion 50% 200% 50% Change religion, Demand provinces (any) $ATTACKER$ Purge of $DEFENDER$ Heresy
Excommunication War Before 1650, Be Catholic, Target is Excommunicated 25% 125% 50% Release vassals, Release annexed, Annexation, Demand provinces (only provinces in same culture group as your nation!), Revoke cores, Become vassal, Reduce sphere, Indemnities, Concede defeat $ATTACKER$-$DEFENDER$ Excommunication War
Trade War Embargoed, Merchant Republics get CB on each other 0% 200% 50% Join trade league, Revoke cores, Indemnities, Concede defeat $ATTACKER$-$DEFENDER$ Trade War
Imperial Ban Emperor, Reichsreform passed, enemy capital outside HRE 10% 200% 50% Release vassals, Release annexed, Demand provinces (only in HRE), Reduce sphere Holy Roman Reclamation of $PROVINCE$
Dishonorable Scum (Badboy Wars) Infamy limit surpassed 25% 100% 75% Release vassals, Release annexed, Annexation, Demand provinces (any), Revoke cores, Reduce sphere, Indemnities, Concede defeat $ATTACKER$-$DEFENDER$ Punitive War
War on Pagans Pagan target, not be a Pagan 20% 100% 50% Annexation, Demand provinces (any), Revoke cores, Release vassals, Release annexed, Reduce sphere, Indemnities $ATTACKER$ Conquest of $DEFENDER_NAME$
Claim Throne Succession claim 10% 200% 100% Form personal union, Annexation $ATTACKER$ War for the $DEFENDER_NAME$ Throne
Tribal conquest (in cb_types.txt under 'Horde vs Civilized') Either attacker has Tribal Despotism, Tribal Federation, or Tribal Democracy government and target is non-Tribal, or attacker is non-Tribal and target has Tribal Despotism, Tribal Federation, or Tribal Democracy government 50% 100% 100% Annexation, Change religion, Indemnities, Become vassal $ATTACKER$ Conquest of $DEFENDER_NAME$

4.1 Beta

The 4.1 Beta has introduced two new Casus Belli in HTTT: "Tribal Feud" and "Horde".


Casus belli Negotiation modifiers
Name Prerequisites/Trigger Infamy Prestige Warscore Affected peace options Name of War
Tribal Feud Attacker borders Tribal target and have Tribal Despotism or Tribal Federation government 25% 100% 100% Indemnities, Annexation, Demand provinces (any), Become vassal, Change Religion $ATTACKER$ Conquest of $DEFENDER_NAME$
Horde One of these must be true: Attacker borders non-Tribal target and have Tribal Despotism or Tribal Federation government, Attacker borders a government with Tribal Despotism or Tribal Federation government and is not Tribal 50% 100% 100% Indemnities, Annexation, Demand provinces (any), Become vassal, Change Religion $ATTACKER$ Conquest of $DEFENDER_NAME$

See also

In Nomine

You can get casus belli (plural) in several ways:

  • If another country owns a province your nation considers one of its cores, you have a CB against that nation. Be aware that core claims go away after a good time. It happens when you lose a core on that province, usually after 50 years or as a consequence of a certain peace resolution. A flag shows up when the CB is about to end.
  • Sometimes an event might pop up giving a CB (a diplomatic incident or a Boundary Dispute). But be careful - you take a huge prestige hit (25 points) any time a core in another land expires. Thus you should only Press The Issue unless you plan to take that province within 50 years. Cores in provinces with your culture, however, never expire. A Diplomatic Insult event gives a temporary Casus Belli that expires within a year from the date received.
  • Should you warn a country and it still goes to war with one of its neighbours you will also gain a CB. (In In Nomine, you get a call to war as if you were an ally of the country attacked)
  • If you are the Emperor of the Holy Roman Empire, then you gain a casus belli on anyone attacking any member of the Empire. (In In Nomine, you get a call to war as if you were an ally of the member attacked. Refusing to defend the Empire seriously damages your chances of being re - elected as Emperor.)
  • If you have Unam Sanctum national idea you have a casus belli on anybody in your religion group who doesn't share your religion. Other countries do not respect this casus belli, however, so you still suffer the full reputation penalty for declaring aggressive war.
  • If you insult a country, it gets a CB against you.
  • When your ally is attacked by somebody or attacks somebody you have a casus belli on that country. (In In Nomine, you will automatically be asked to join the war of any ally that gets attacked. Refusing to join in the defense of your ally dissolves the alliance and gives them a casus belli against you.)
  • If you send a spy to a capital city, you can fabricate claims on their country. If the spy is successful, you gain a casus belli on that country for a year.
  • If you have a royal marriage, you can try to claim their throne. You gain a CB on that nation but suffer a penalty in relations with every other nation you have royal ties with, but you still suffer the -1 Stability hit for attacking Royal Relatives.
  • If you offer a loan to a country and the country does not pay the loan you will be given an casus belli. However many players believe that the loan offer is an exploitative casus belli because countries rarely repay the loan .

There are some changes to how religions influence CBs in In Nomine:

  • For the early part of your game you essentially have a free CB against all infidels—that is, a Catholic nation could attack a Muslim one with neither a stab hit nor a BB penalty. The war would still be titled the nth war of [Country] Aggression, though.
    • This bonus goes away after the end of the counter reformation, in the mid 17th century
    • The Unam Sanctum NI is much less valuable early game for this reason, but still seems to provide a partial amelioration of penalties when attacking heretics (e.g., Protestant versus Catholic)

Military:

Leaders  • Maintenance  • Morale  • Manpower  • Ship types (Big ship  • Light ship  • Galley  • Transport)  • Land unit types (Infantry  • Cavalry  • Artillery)  

Warfare:

Assault  • Basic Warfare  • Casus Belli  • Combat • Making peace • Siege • Supply & attrition • War Capacity  • War Exhaustion