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.
- 1 Starting a war
- 2 Sides in a war
- 3 Fighting a war
- 4 War Exhaustion
- 5 Warscore
- 6 Sue for peace
- 7 Negotiating Peace
- 8 Strategies and tactics
- 8.1 Declaring war
- 8.2 Types of war
- 8.3 Proxy wars
- 8.4 Deliberately breaking a truce
- 8.5 Using subjects to annex provinces
- 8.6 Separate peace
- 9 Footnotes
Starting a war
Like most diplomatic actions, declaring war requires a diplomat. War may not be declared on an ally, a subject or a guaranteed nation without first breaking that relation.
- 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.
No casus belli
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.
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 +50% aggressive expansion and costs +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 called also
- Taking that country's provinces will cost the same as the war leader's provinces
Sides in a war
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.
The warleaders are denoted with a star in the war screen, the warleader on the attacking side is the country that declared the war, while on the defending side - 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.
Military and port access
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
After a war has started, a country usually cannot join any of the sides unless called in later. There are 2 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. 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
War may be found on both the land and the sea. See the appropriate articles for more information.
- Main article: War exhaustion
War Exhaustion represents the will of a country's population to fight. A high War Exhaustion will sap the ability of a country's armies to fight and reinforce.
Call for peace
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, 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.
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 EUIII, EUIV, Crusader Kings II and Victoria II.
The scale ranges from 100 (a complete victory for the offending side) to -100 (complete victory for the defending side).
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 won 80% of battles, and a province goal won't tick if a third party (e.g. rebels) controls it.
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 province 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 and trade ranges.
- The controller of the province can recruit mercenaries there.
- Occupied forts project a zone of control over ally-controlled land. Likewise, occupied unfortified provinces are affected by zones of control of nearby allied forts.
- 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 60%. Administrative efficiency directly reduces core creation and diploannexation costs, also the impact of province development on overextension and warscore cost, allowing for much larger territories to be conquered at once.
Sue for peace
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.
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. The AI does not offer unconditional surrender. They are however able to accept them.
Each term in a peace offer has an associated cost in warscore, Prestige gain/loss for both sides and possible aggressive expansion for anyone making sizable, selfish demands. An appropriate casus belli may modify any or all of these values, and is required to enable certain terms.
|Annex province||Province cost||5||15||The winner annexes the selected province of the loser. It must (eventually, after coring other provinces given in the peace) be possible for the annexing country to core this province, if it is not already a core. It must also not be occupied by a third party; if it is not occupied, it costs 10% more warscore (20% for capitals). In a coalition war, members of the coalition may only take cores. AI won't accept giving away unoccupied forts, or unfortified provinces that are near forts if none of them are occupied.|
|Revoke core||Half province cost||1||0||The loser revokes their core on the selected province that they didn't own before the war. Revoking cores of third parties gives them a small relations bonus towards the winner. Never costs diplomatic power.|
|Return core||Province cost||1||0||The loser returns the selected core province of an existing country, and loses their core if the province is not of their primary culture.|
|Give up claims||20%||2 per province||0||The loser revokes all claims on the selected country, who need not be involved in the war. If it's a third party, this gives them a small opinion bonus towards the winner.|
|Cancel subject||Half of release nation cost||5||0||The selected subject of the loser becomes independent.|
|Release nation||2% + sum of province costs||1 per province||0||A new nation will be formed out of the loser's provinces with cores of that nation. The new nation will be included in the resulting truce. The new nation will have its capital's religion and culture. The loser also loses their cores except on provinces of their primary culture.|
|Force religion||Sum of province costs||10||0||Loser converts state religion to that of the winner. Only possible within the same religious group.|
|Form Personal Union||60%||20||30||Loser becomes lesser partner of a personal union with the winner. Only available with Claim on Throne and Restore Union CBs.|
|Become vassal||Sum of province costs||2 per province||15||Loser becomes a vassal of the winner.|
|Pay tribute||1% per month||0.1 per month||0||Loser pays an immediate lump sum of to the winner, measured in months of income. Limited to treasury plus one loan, and results in inflation for the winner.|
|Concede defeat||10%||10||0||A "white peace plus", the only change being a larger gain and loss of prestige than other peace deals such as paying gold (±10 instead of ±2). Not compatible with any other terms.|
|Annul treaties||10%||1||0||Loser cancels all treaties (including military access) with the selected country, and is not allowed to sign any new treaties with that country for a duration of 10 years. Royal marriages are unaffected.|
|Change government||50%||Loser's government type changes to that of the winner. Only available with Government Type and Revolutionary War CBs.|
|Transfer trade power||60%||1||0||Establishes a "Transfer Trade Power" relationship from the loser to the winner, amounting in 50% of the trade power in all trade nodes where both countries have trade power. This lasts until cancelled, which the loser can't do until the truce expires. As of patch 1.16 this no longer occupies a relationship slot.|
|Humiliate||40%||5||0||Winner gains 30 power projection, loser loses 30 power projection. Note that the power projection bonus from humiliating any one rival caps out at 30; and doesn't stack, instead refreshing the bonus.|
|Show strength||100%||0||0||Winner gains 30 power projection + 100 of each monarch power, loser loses 30 power projection and 20 prestige. Note that the power projection bonus from humiliating any one rival caps out at 30; and doesn't stack, instead refreshing the bonus.|
|War reparations||10%||2||0||The losing party is forced to pay 10% of their income to the winning party as war reparations. This lasts for 10 years.|
|Establish protectorate||1% by province wealth||0.05||0.25||Establish a protectorate on a nation. Only works when the difference in technology group is large enough.|
|Revoke elector||60%||5||25||Revoke elector title.|
|Force migrations||100%||2||0||Force a primitive nation to migrate elsewhere. Only possible when the nation is small enough.|
|Enforced military access||15%||0||0||Enforce a nation to give military access until truce expires.|
|Enforced fleet basing rights||25%||0||0||Enforce a nation to give fleet basing rights until truce expires.|
|Enforce rebel demands||50%||2||0||Enforce a nation to accept the rebel demands. only works with the support rebels casus belli.|
- Annex province
- Cancel vassal
- Release nation
- Force vassalization
No single entry of a peace treaty can cost more than 200 diplomatic power (for example, releasing a huge junior PU partner won't cost more than 200 diplomatic power).
The diplomatic power cost of unjustified demands can be reduced by:
- Despotic Monarchy: -10%
- Demanding provinces from a rival: -33% for annexing provinces only
- Having a claim: -10%
Distribution of spoils
When playing with the cossacks DLC, spoils of war are now distributed between its participants with prestige and money going to the belligerents of the war based on their war contribution rather than only to the war leader. For example, if Spain and France fight in a war against Great Britain and France gets 70% war contribution and Spain 30% then the prestige and money will be divided so that 70% goes to France and 30% goes to Spain.
Once a peace deal is accepted, it results in a state of truce between every country on the offering side and every country on the accepting side. Specifically, when a country other than a war leader accepts peace, or offers a peace deal the other side's war leader accepted, it enters truce with every other country it fought in the war, but countries on its original side are not affected. A country cannot embargo or join a coalition against a country it has a truce with. It can still declare or join a war with the country with a truce, but with the following consequences: Breaking a truce will result in the following penalties:
|Ideas||Stability||War exhaustion||Aggressive expansion|
|Normal||-5||+5 war exhaustion.||-50|
|Full Diplomacy||-3||+3 war exhaustion.||-30|
These stack with the penalties for other reasons. So attacking without a CB and without full diplomacy results in -7 stability, +7 war exhaustion and -70 aggressive expansion. As seen above, breaking a truce has very negative effects when declaring war or joining an offensive war against a country with whom a truce exists. The AI has -1000 reasons to join an offensive war if they have a truce with the defender and appears to never declare war or can in an ally in an offensive war if that would break a truce. Joining defensive wars does not give a penalty. The duration of the truce depends on the amount of warscore that was used to demand or offer tribute in the peace deal. The formula is
so a white peace will result in a 5-year truce, whereas a full annexation at 100% war score will produce a 15-year truce. When a country is released in a peace deal, it starts with a 5-year truce with both the releasing nation and the opposing war leader.
Revanchism is a mechanic that helps prevent a "death spiral" after losing a war where a country would go bankrupt, lose manpower and have huge revolts, followed by other countries declaring war as well, leading the nation to ruin. Revanchism is gained from losing provinces in a war and scales in direct correspondence to the war score taken in a peace deal, with a max of 100% revanchism at 100% warscore. Once at 100% revanchism it takes 20 years for the effect to fully wear off, lasting longer than the resulting truce that caps out at 15 years for 100% warscore. Releasing nations and returning cores does not grant revanchism.
With maximum revanchism of 100% (from having 100% warscore worth taken in a peace deal), a country gets the following modifiers: 
|+50%||National Tax Modifier|
|+50%||Manpower Recovery Speed|
|+1%||Yearly Army Tradition|
|+1%||Yearly Navy tradition|
|-2%||Interest per annum|
|+10||Yearly horde unity|
These modifiers scale linearly with revanchism from 0% to 100%
Note that revanchism caps out at 100%, even if a peace deal worth more than 100% warscore in total was taken (such as in multiplayer that allows peace deals over 100% warscore or from multiple peace deals).
Human players are largely free to accept or refuse any peace deal. However, as with other diplomatic actions, the AI will accept a peace deal if and only if it has more positive than negative reasons to do so.
The warscore is of primary importance. Each point of warscore will give +1 reason for the AI to accept a peace deal, whereas each warscore cost of the peace terms will give -1 reason for the AI to accept a peace deal.
Demands exceeding warscore will give additional negative reasons for the AI to accept the peace deal, up to -100 at 99% warscore and lower. Demands exceeding 100% warscore will give -1000 reasons (unreasonable demands, not to be confused with Unjustified demand). Individual demands that would cost over 100% warscore, such as vassalizing a country that is too large, can't be selected.
Regardless of warscore, the AI may refuse peace deals that contain terms that they do not desire; for example, provinces they will be unable to core.
Each province that changes control in a peace deal requires a corresponding cost in warscore. The factors that influence each province's warscore cost are:
- Base: 5 per province.
- Tax value: 2 per province base tax.
- Manpower: 0.03 per province manpower, rounded up to the nearest integer.
- Trade power: 0.2 per base trade power.
- Fort level: +5% for each fort level beyond 1.
- Local autonomy: -0.33% per percentage point.
- Size of nation: -1% per 15 base tax of nation.
The local autonomy, and size of nation factors are applied multiplicatively.
Other factors also give reasons for the AI to accept or refuse a peace deal. These are summed up as war enthusiasm in the war screen. Positive War Enthusiasm is applied as negative reasons for that country to accept peace deals.
- Length of war: +45 at start of war, decays by -0.75 at the start of each calendar month thereafter
- Relative strength of alliances: -20 to +20 for war leader only
- Ally in war: +10 for allies called in by war leader
- Military strength: -20 to +20, depending on proportion of manpower and land force limit that is filled, as well as battles lost or won
- Recent gains: -20 to +20 for war leader only. Note that due to a bug, it can be possible to get up past +20 temporarily, but it will be reset to +20 at the beginning of every month.
- Hold their own capital: +5
- War exhaustion: -1 per point of war exhaustion
- Occupied and besieged provinces: scales linearly with base tax of provinces affected, up to -20 for full besiegement, and -40 for full occupation. The formula is roughly
- Colonies and distant overseas provinces are ignored in the calculation above.
- Revolts in country: depends on the size of all rebellions relative to current military strength. The formula is
- with the final number rounded to the nearest integer and capped at -20.
If war enthusiasm is between 0 and 20 it's classified medium, below it is low and above it is high.
Strategies and tactics
"War cannot be avoided; it can only be postponed to the other's advantage." - Niccolò Machiavelli
To understand war and ultimately be victorious, it is necessary to differentiate between the reason a war broke out, and its cause. The reason is an event that happened recently, a short term effect of some sort. The cause of the war is the underlying conflict, a long-term situation that needs to be resolved, often by warfare.
Reasons for war
In Europa Universalis IV the most common reason for war is that a country finds the military capabilities of their enemies inferior to their own. When a certain ratio is met, nations will declare war on others, seizing the opportunity of weakness. Another reason can be a shift in diplomatic relations, such as through border friction or allying with a rival of another ally, which can turn even close friends against each other. To prevent reasons from occurring, is to prevent a war breaking out. Notice that this applies even when nations are already at war - if anyone seems weak enough, enemies who would usually not, would join a war against them.
Cause of war
Every nation has a long-term agenda; to understand this agenda is to understand the cause of war. Two nations situated on a confined piece of land, as with Scotland and England, will inevitably clash in conflict, even though this conflict can be resolved in different ways: annexation, vassalization, personal union or through the appearance of a bigger threat which can lead to an alliance until this threat is over, such as an expansionist France.
To deduce the long-term agenda of a country, the diplomatic view is essential. These will change with the diplomatic landscape, but will not be affected by minor changes. Nations should actively try to manipulate and control the change of this landscape, joining alliances and coalitions, siding with certain countries at certain times. As a general rule, each country seeks to become more powerful in the long run. Powerful countries will become competitors, but their competition may be continuously delayed by other circumstances. Controlling these circumstances means to control the relations between countries.
Everyone is a potential enemy. Constant attention and care to the diplomatic landscape and agendas of countries is needed to know of the potential causes. The one who knows the causes can predict future conflicts.
Types of war
"War is not merely an act of policy but a true political instrument, a continuation of political intercourse carried on with other means." - Carl von Clausewitz
Even wars will (usually) not be fought just for fun, but to achieve a political goal. Despite the multitude of Casus Belli, countries will mainly be involved in only three types of wars. Each type defines with which goal the war is started, but not necessarily how the war is won. For strategies and tactics to win a war, see Strategies and Tactics for Land Warfare or Naval Warfare.
A war where the goal is for the country to defend itself from an enemy attack.
A war where a country seeks to conquer their enemy's territory.
A war where the country wants to force their enemy to convert to a certain religion.
War for trade power
There might be situation where it is profitable to fight a war to obtain part of an enemy's trade power. However, a war will always be costly, and the enemy's trade power will only be transferred for the duration of the peace treaty. The advantage of such an undertaking is that only 30% warscore is required, and an opportunity to weaken a rival is provided.
War to weaken an enemy
It can be reasonable to start or join a war with the only goal of weakening an enemy, be it by making the enemy waste manpower, release provinces to third parties or as new nations, or cancel subjects or alliances. This has the advantage that only a small warscore will be required to end the conflict through white peace or a minor demand for which no diplomatic power will be needed, and no aggressive expansion or overextension will result.
This war can serve three purposes: First, a weaker enemy is a less dangerous enemy. If they exhausted their manpower pool, it will take them many years to return their armies to full strength. Second, their weakness can lead to others seizing the opportunity and fighting the war of annexing provinces. When used with caution, it is possible to shape the political landscape and shift the balance in or out of a country's favor. Third, it may make future actions feasible that weren't before, due to excessively high warscore, diplomatic cost or overextension. Weakening the enemy is the essence of the "divide and conquer" strategy.
Another method to achieve the weakening of an enemy is to solely conduct a proxy war.
Enforcing Peace or to protect future interest
It's the only casus belli that is not a casus belli. For example, when a ruler wants to protect a weak neighbor so they can be vassalized, annexed, or used as a buffer in the future, it's probably a good reason to enforce peace. Another fine reason for enforcing peace is when it is beneficial to not let either side of the war benefit from it. What happens when peace is enforced is that the nation at war with the nation that is getting attacked has to decide whether it's better to continue the war with the new extra enemy (the enforcer of the peace), or to lay down arms. If the second option is chosen, a white peace will be signed between the two parties.
Warnings and guarantees can serve similar purposes. Since a warner or guarantor will be called into the war, the enemy must take this into account when preparing for war, and may decide not to declare war at all. But remember that a formal guarantee to protect a nation which is planned to be annex later has to been done with care (as opposed to simply diplomatically vassalize), since revoking the guarantee results in a truce.
Every so often rebels, pretenders, nationalists, patriots, religious zealots and others will try to disturb the peace within a country. Although they often come with only one army at a time and do not cooperate like a nation or coalition would do, mass uprisings can lead to a state of virtual war. These rebels come in different strengths, but their morale will always be comparable to or less than the soldiers of their overlord. These insurgents often have one specific goal of conquering certain provinces. Since it takes many months to win a siege and rebels will not actively seek out other armies, it is possible to stall for morale regeneration and reinforcements before engaging to break the siege.
Every time a nation actively supports a belligerent faction through indirect means, that qualifies as a proxy war. Examples include war subsidies, cheap loans, gifts, and renting out condottieri. The second method of conducting a proxy war is to support rebels. This can be especially effective when the enemy has provinces that are hard to defend, like islands or exclaves.
The main advantage of a proxy war is that one's nation is not involved directly, so the official diplomatic status and relations will be largely unchanged. Notice however that it is possible for a human player to notice such actions, as paying subsidies is visible for everyone, and diplomats being uncovered will reveal one's involvement and worsen relations.
Of course, the above methods can be mixed with classic warfare: It is possible to support war allies with money, while at the same time supporting rebels in enemy territory to further weaken them, while also militarily attacking them. Note that provinces held by supported rebels will not affect warscore. However, it can lead to the rebels forming a new nation which will be allied with their supporter. This will simultaneously decrease the enemy's total number of provinces, making warscore easier to gain as fewer provinces will have to be sieged.
Deliberately breaking a truce
Breaking a truce is costly, but sometimes a necessary action to take. War exhaustion will be increased and stability will be lowered. When planning on breaking a truce, a nation should have enough administrative power to boost their stability up again. Truces should only be broken when not intervening would result in dramatic consequences, such as a personal union between two very powerful nations.
Using subjects to annex provinces
Provinces gained through peace deals will yield overextension, aggressive expansion penalties and eventually the need to core it. To avoid the coring and overextension costs, provinces can be sold or given (requires DLC) to subjects. This is only possible when the subject is not overextended, is able to core the province and is not currently being integrated. It is also possible to directly release a new vassal from newly annexed provinces, which will reduce overextension, aggressive expansion and the need to core it.
It is a good idea to sign separate peaces with minor parties in a war for a few reasons:
- It removes their troops from the war, making the war leader more likely to agree to peace, and reducing opposition
- It is not possible to negotiate a peace for more than 100 warscore, thus using separate peaces can lead to higher total gains
Be wary of raising overextension too high when doing this. It is entirely possible to get total overextension over several hundred percent when signing several separate peaces in quick succession. If possible, a country should focus on the secondary combatant at the beginning of the war. This will allow to take them out sooner and begin the coring of their newly annexed provinces earlier. By the time the war is over, the lands will be cored and new provinces can be taken from the war leader.
Note: this does not work against coalitions as their members can not sign a separate peace. Their provinces, however, can be taken when signing peace with the war leader.
- See in Static modifiers#Revanchism). under recovery_motivation(