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EU4 Beginner Tutorial #4 - War. A complete beginner tutorial with MordredViking looking at how warfare works in the game.

Warfare is one of the primary ways to obtain territory and other concessions from other nations. The technical aspects of maintaining a military machine and its employment on the field is discussed in-depth in the articles land warfare and naval warfare.

Starting a war[edit | edit source]

Like most diplomatic actions, declaring war requires a diplomat. War may not be declared on an ally, a subject, a guaranteed nation or a nation transferring you commercial power without first breaking that relation.

Casus belli[edit | edit source]

Main article: Casus belli

The aggressor may pick a casus belli when declaring war. This determines the wargoal, the options available in the subsequent peace deal, and their associated costs in warscore, aggressive expansion, and diplomatic power.

Declaring War with a Casus belli

No casus belli[edit | edit source]

Declaring war without a casus belli will result in -20 Aggressive expansion impact.png aggressive expansion, a -2 drop in Stability.png stability, and +2 War exhaustion.png war exhaustion. These penalties are halved with full Diplomatic ideas.

Good relations[edit | edit source]

A country declaring war on another whose opinion of them is higher than 100 will cause -1 stability and +1 war exhaustion. If opinion is higher than 150, penalty increases to -2 stability and +2 war exhaustion.

Co-belligerence[edit | edit source]

If a country is not marked as a co-belligerent:

  • That country can't call its allies to arms in that war (but its subjects are called as normal). Beware that the Holy Roman Emperor defending the empire is always a co-belligerent.
  • Taking that country's provinces in a peace incurs Aggressive expansion impact.png +50% aggressive expansion and costs War score.png +100% warscore.

If a country is marked as a co-belligerent:

  • That country can call on its own allies to fight as well
  • If that country has a guarantor, it is also called to war
  • Taking that country's provinces will cost the same as the war leader's provinces

Note: If a nation is marked as a co-belligerent, but are also a tributary of another nation, neither the tributary overlord, nor the co-belligerent's allies will be called in. A similar loophole also applies in the HRE. If a free city or an HRE member is marked as a co-belligerent neither the Emperor nor the Emperor's allies will be called in. But in this case the allies of the co-belligerent will be called.

Sides in a war[edit | edit source]

A war consists of two opposing sides of one or more countries. A side may be made up of a coalition, the emperor, allies or the defender of the Faith who honour their treaties and/or vassals and other subject countries.

War leader[edit | edit source]

The war leaders are denoted with a star in the war screen. The war leader on the attacking side is the country that declared the war, while on the defending side the leader is the country on which the war was declared (the target of the war goal). For each side in a war the leader is the country that may call in its allies and may negotiate peace on behalf of all their war allies, simultaneously ending the war for everybody. They may negotiate a separate peace with each of the hostile belligerents, except subject nations or coalition members. They may surrender territory of their war allies, but not their treasury.

When a war leader is annexed in a separate war, one of their allies will become the new war leader. The new war leader can call their own allies into the war if the war has not lasted long enough to close the window for calling allies to it. If the war leader is vassalized (by event or force) the new overlord will become the new war leader and can call their allies with the same caveat about calling allies to long wars.

Military and port access[edit | edit source]

All nations on the same side in a war will have immediate military access to each other's lands. They may also dock at each other's ports, although their fleet supply range will not be extended. Furthermore, any nation in a war will be able to walk through all nations that have given access to any of the belligerents, even if those are not participating in the war.

Joining a side[edit | edit source]

After a war has started, a country usually cannot join any of the sides unless called in later. There are three exceptions to this rule. The enforce peace action allows for a country to force the attacking war leader to sign a white peace. If the attacker does not accept, the enforce will join the war on the defender's side. Another way to join a war after it has started is for a country to vassalize one of the countries involved in any of the sides. This will drag the new overlord to war as well. Finally, with Rights of Man.png Rights of Man, a great power may intervene in a war involving at least three other great powers on the side that has fewer great powers.

A way of indirectly fighting in a war is for a country to rent out condottieri to one of the countries involved.

Fighting a war[edit | edit source]

Main articles: Land warfare, Naval warfare

War may be fought on both the land and the sea. See the appropriate articles for more information.

War exhaustion[edit | edit source]

Main article: War exhaustion

War exhaustion represents the will of a country's population to fight. High war exhaustion will sap the ability of a country's armies to fight and reinforce.

Call for peace[edit | edit source]

If the warscore is substantial (66.6%+) and it has been at least 5 years since the war was declared, a country's abstracted population may call for peace. This modifier increases the nation's monthly War exhaustion.png war exhaustion, beginning at +0.01 per month, and ticks up by another +0.01 per month indefinitely. This means a call for peace will eventually start increasing a country's war exhaustion even if it has a monthly reduction, e.g. from being Defender of the Faith, having the 6th Innovative idea, or having the Kind-Hearted ruler trait. Only human players get call for peace.

Call for peace will also occur if the country fails to submit a peace offer within 3 months of an enemy surrendering unconditionally. In this case the ticking war exhaustion begins at +0.05 per month and increases more quickly.

Warscore[edit | edit source]

Warscore is the means of measuring whether a war is going in the favor of the aggressor or the defender. It is a common metric used across a number of Paradox Interactive titles, including Europa Universalis III, EUIV, Crusader Kings III and Victoria II.

The scale ranges from +100% (a complete victory for the side currently being viewed) to −100% (complete defeat for the side currently being viewed).

Warscore is measured using a number of different parameters:

  • Occupied provinces
  • Battles won or lost, to a maximum of 40% in either direction.
  • Blockaded ports
  • Met war goals. A met war goal will cause war score to gradually tick up for whichever side has met it, to a maximum of 25%.

Note that both sides have symmetrical war goals. It is possible for no one to have met the goal; in particular, Show Superiority warscore won't tick if neither side has more than 10% warscore from battles, and a province goal won't tick if a third party (e.g. rebels) controls it. Hence, for provinces that are being colonized and yet are the war goal, rather seize them for yourself during the war, otherwise the enemy may destroy or abandon them and the target will then cease to exist – unable to provide a ticking war score.

Occupation[edit | edit source]

Occupation is the term used to describe when a province has been successfully taken over by an enemy country. It requires the successful siege or assault of the local fortification. Upon occupation of a province, the owner of the province can no longer use the province for many purposes:

  • Regiments, ships and buildings can no longer be built in the province, and any unit or improvement building that was underway (including cores and religious or cultural conversion) is immediately halted.
  • All province income and trade power is no longer given to the owner; a portion of the province's production income and trade power is now given to the occupier.
  • The owner can no longer use the province for a fleet base; any ships currently in the port are forced out to the adjacent sea zone. The controller does have access to these ports.
  • The province does not count for the owner for calculating naval supply, colonial range and trade range. This is important, as one may have to unsiege certain provinces far away in order to take provinces in a peace deal (due to lack of colonial range). Force limits are unaffected.
  • The controller of the province can recruit mercenaries there.
  • Occupied forts project a zone of control over unfortified land owned by countries that aren't allied to their occupier.

Transfer occupation[edit | edit source]

Countries can give up control over occupied provinces to their allies in war. (The Transfer occupation.png ‘transfer occupation’ button is located on the province screen.) This will make it possible to ensure that nations are rewarded for their participation in the war.

If you called an ally in with a promise of land, you may often find in the peace deal that they "do not want" the province in question - even if they highlighted it as a "province of interest" in the Diplomatic Feedback window. Hence, they will lose trust in you if you take any land or war reparations. However, there is a solution: before you peace out, make sure that ALL the land they highlighted as "province of interest" are under their control (hence you have transferred occupation to them), then even if you give them nothing, they will not lose trust in you (shown as a thumbs down warning in the peace menu, instead it will be neutral).

Administrative efficiency[edit | edit source]

Main article: Overextension#Administrative efficiency

Administrative efficiency is a country wide bonus that is unlocked at administrative technology level 17 and increases at 23 and 27, up to a total of 30%. Additionally Absolutism.png Absolutism provides additional administrative efficiency scaling with amount, up to 30% at 100. Administrative efficiency directly reduces core creation and diplomatic annexation costs, as well as the impact of province development on overextension and warscore cost, allowing for much larger territories to be conquered at once.

Sue for peace[edit | edit source]

Peace interface, on the "Demand tribute" tab.

This option will open the peace negotiation screen where the country will negotiate their demands, the terms of their surrender or simply a white peace. The leader of each war alliance can make peace separately with each independent country on the other side (except any that joined as part of a coalition), in which case only individual warscore against that country and its subjects is taken into account (battles only count towards overall score). This can be essential to get the desired peace deal - the overall warscore may be lower than against a single participant, so the country can get more out of the war by picking off participants one by one.

Unconditional surrender[edit | edit source]

Upon offering unconditional surrender, all of the currently unoccupied provinces will fall under enemy control and the enemy will gain 100% warscore. Armies of the country that surrendered will become exiled and unable to fight in future battles until peace is signed. For the recipient of an unconditional surrender, it will be alerted of the enemy’s surrender and from then on will be able to enforce any possible peace up to 100% warscore cost. If the recipient country does not sign peace after a couple months, they will get call for peace giving them monthly war exhaustion which increases faster than normal. The peace will be automatically accepted by the nation that surrendered. In some circumstan