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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]

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.

Casus belli[edit]

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]

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

Good relations[edit]

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 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 called also
  • Taking that country's provinces will cost the same as the war leader's provinces

Sides in a war[edit]

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.

When a warleader is annexed in a separate war, one of their allies will become the new warleader. The new warleader 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 warleader is vasallized (by event or force) the new overlord will become the new warleader and can call their allies with the same caveat about calling allies to long wars.

Military and port access[edit]

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]

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[edit]

Main articles: Land warfare, Naval warfare

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

War Exhaustion[edit]

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[edit]

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 Icon 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.


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, Victoria II, and Stellaris.

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 more than 10% warscore from 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.

Transfer occupation[edit]

Countries can now 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.

Administrative efficiency[edit]

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 provides up to another 40% Administrative Efficiency at 100. 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[edit]

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]

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.

Peace terms[edit]

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.

Demand Warscore Prestige Aggressive Expansion Effect
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). If it is occupied by a country other than the one making peace, its current controller annexes it. 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.
Concede Colonial Area Province cost × 1.33Unconfirmed 5 5 The loser (or their colonial subject) cedes all provinces in the selected area to the winner's colonial subject. Furthermore, for the duration of the truce, the loser may not establish any additional colonies in the region.
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 Restoration of Union CBs.
Become vassal Sum of province costs 1 per 2 development 15 Loser becomes a vassal of the winner.
Become tributary Sum of province costs 1 per 2 development Loser becomes a tributary of the winner.
Pay tribute 1% per month 0.1 per month 0 Loser pays an immediate lump sum of Gold Icon.png ducats 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 (unused) 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. Transfer Trade Power does not occupy 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. Only available if the loser is the winner's rival.
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. Only available with the "Humiliate Rival" casus belli.
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.
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.

Unjustified demands.png Unjustified demands[edit]

Peace terms that are not part of the wargoal will cost 3Diplomatic power.png per development of affected provinces:

  • 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:

Unjustified demands.png Traditions Ideas Bonuses Policies
  • Full Influence
  • Creek traditions
  • Kikuchi traditions
  • Siberian traditions
  • Moravian idea 7: Legacy of Great Moravia
  • Imagawa idea 7: March to Kyoto
  • Despotic Monarchy: -10%
  • Ottoman Government: -10%
  • Demanding provinces from a rival: -33% for annexing provinces only
  • Having a claim: -10%

Distribution of spoils[edit]

Shows distribute spoils in game.

Spoils of war are 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 Icon stability.png Stability Icon war exhaustion.png War exhaustion Aggressive expansion impact.png 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

\text{peace duration in years} = 5\,+\,0.1\,\times\,\text{percentage of war score the peace deal is worth}

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.


An example of revanchism from an EUIV developer diary.

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: [1]
National tax modifier.png +50% National tax modifier
Fort defense.png +25% Fort defense
Manpower recovery speed.png +50% Manpower recovery speed
Army tradition.png +1% Yearly army tradition
Navy tradition.png +1% Yearly navy tradition
National unrest.png -5 National unrest
Interest per annum.png -2% Interest per annum
Horde unity +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).

Negotiating peace[edit]

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.

Province cost[edit]

Each province that changes control in a peace deal requires a corresponding Province warscore cost.png warscore cost. The factors that influence each province's warscore cost are:

Base 5 per province.
Development 1 per development, cap at 30
Trade power 0.2 per base trade power (before modifiers applied)
Capital +2% of (base + development) cost if the province is a capital
Local autonomy −0.33% per percentage point
Size of nation −1% per 15 development of nation (owner)
Administrative efficiency −1% per point of of the annexing country's Administrative efficiency.png administrative efficiency

The local autonomy, size of nation and administration efficiency factors are applied multiplicatively.

The following ideas reduce province warscore cost:

Province warscore cost.png Traditions Ideas Bonuses Policies
  • Diplomatic idea 6: Flexible Negotiation
  • Desmondian idea 4: The Munster Ambition
  • Ito idea 4: Anti-Shugo Movement

War enthusiasm[edit]

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. Some casus belli increase this by +50 (in an unmodded game, Religious League and Crush the Revolution).
  • 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
\text{modifier} = -\frac{(20 \cdot \text{base tax under siege}) + (40 \cdot \text{base tax occupied})} {\text{total base tax}}
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
\text{modifier} = \frac{-20 \cdot \text{number of rebel regiments on owned territory}} {\text{number of national regiments}}
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.

Stability hit[edit]

If a player refuses a peace offer that entails demands for less than 50% of the current warscore, the declining party takes a -1 hit to Icon stability.png stability. When a player is already at -3 Icon stability.png stability, any qualifying offer is automatically accepted on behalf of the player.

The AI will almost never refuse an offer that would result in a stability hit, so its main area of application is multiplayer games, where this mechanic aims to prevent players from unreasonably dragging out lost wars to harm his/her opponents (by deliberately causing war exhaustion to rise or rebels to spawn).

Strategies and tactics[edit]

Declaring war[edit]

"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[edit]

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[edit]

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[edit]

"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.

Defensive wars[edit]

A war where the goal is for the country to defend itself from an enemy attack.

Conquering land[edit]

A war where a country seeks to conquer their enemy's territory.

Force conversion[edit]

A war where the country wants to force their enemy to convert to a certain religion.

Other wars[edit]

War for trade power[edit]

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[edit]

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.

War to strengthen Imperial Authority[edit]

If the Holy Roman Emperor wishes to pass reforms, he needs to gain Imperial authority.png Imperial Authority. The two main ways to do this are to add provinces to the empire and to simply accumulate it over time.

To some extent, it is possible to increase authority peacefully. The main tool for this is the diplomatic action "Demand Unlawful Territory". If a country has annexed a prince, but not yet cored all of its conquests, it may accept such a demand and return a province to a country that doesn't currently exist, thus creating a new prince. This comes at the cost of a −100 opinion penalty, so take care using it on electors, or you may find yourself losing the next imperial election. A single term for a weak emperor in the face of a rampaging France, Poland, Denmark/Sweden or England can be even worse for authority than losing a prince.

However, countries are naturally jealous of their conquests and so more often war is necessary to persuade countries to spit out annexed countries. The aim of such a war is only to gain land for the emperor if no HRE member has, or alternatively will have, a core on it; normally it is better to save on Aggressive Expansion by using "Return Core" to a prince, or "Release Country" if the new country's capital will be in the empire, especially if the aim is to release a one-province republic that will accept an offer to become a Free City. Although owning a 10-development province directly gives more land force limit and manpower, releasing a prince is often a better tradeoff between those values and Imperial Authority, Aggressive Expansion, and administrative power. "Demand Unlawful Territory" is still a valuable tool here, since a refused request gives the province an "Unlawful Territory" modifier that bestows the "Imperial Liberation" casus belli that continues even after it's cored. Still, there are a number of cores within the empire that start the game owned and cored by another country (e.g. Flag of Parma Parma); these must usually be released via another CB, such as Conquest or by attacking their ally.

The other situation in which war is needed to increase Imperial Authority is when provinces are owned by a prince, but that prince is a subject of a non-member. Two such situations exist at game start: the Dutch minors under Flag of Burgundy Burgundy, and Flag of Holstein Holstein under Flag of Denmark Denmark. There is no way to do this peacefully, so one must attack these countries eventually.

A further concern is the Shadow Kingdom event. There are a potential twelve princes in northern Italy even without conquering any land from the Papal State or Venice; with them, it rises to sixteen (including Urbino and Aquileia). Losing them all is therefore very undesirable. An early focus of any would-be emperor should therefore be to get these areas under the Emperor's control. Two approaches are possible: annexation or vassalization (except for Brescia, which can be returned to Milan). Either way, with a total of 136 development to be brought into the empire within the space of just over 45 years (the deadline for the Rein in Northern Italy decision is 1490), careful planning is needed to avoid too much Aggressive Expansion.

One last type of war under this category is more properly a war to avoid losing Imperial Authority. Refusing a call to arms to defend the empire costs 10 Imperial Authority, so such calls should always be honoured if they would not be disastrous.

Enforcing Peace or to protect future interest[edit]

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.

Civil war[edit]

Every so often rebels, pretenders, separatists, 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.

Proxy wars[edit]

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[edit]

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.png 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. It is also possible to 'break' a truce by attacking an ally of the primary target (a minor who got a separate peace, or a guarantee) even if you only white peace your primary target, you can then turn a 15 year truce into a five year truce.

Using subjects to annex provinces[edit]

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 The Cossacks.png 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.

Separate peace[edit]

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.

Declaring war on the ally instead of the actual target[edit]

Often, the target you want to attack will have a multitude of allies, turning an easy victory into a protracted one or a protracted, but more or less certain, victory into much more of a gamble. Of course, the nations that benefit the most from this tend to be either powerful or wealthy, which often makes them more desirable to attack than a weaker enemy. Additionally, you occasionally want to attack a country that you cannot get a CB on, such as an HRE prince with releasable cores that aren't unlawful territory.

One way around this is to declare war on an ally of theirs, ideally one which is significantly weaker and has fewer allies of their own; when your de facto target joins in on their side, you can fight the war normally, but largely ignore the "primary" target when the time comes to negotiate peace.

The obvious limitation to this is that (since you cannot declare the de facto target a co-belligerent without defeating the purpose of the strategy) you will have to pay an inflated warscore cost for the provinces you demand. However, this is often a relatively small price to pay; moreover, since enemy nations are typically more willing to concede their allies' provinces than their own, you may be able to demand the same amount at a relatively low warscore as long as you don't sign a separate peace.


  1. See in /Europa Universalis IV/common/static_modifiers/00_static_modifiers.txt under recovery_motivation(Static modifiers#Revanchism).
Land warfare ArmyLand unitsDisciplineManpower
Naval warfare NavyNaval unitsSailors
Other concepts Casus belliWar exhaustionMilitary traditionLeadersAlliance