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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.

War is a diplomatic state between two or more countries, in which conflict exists between them. It is one of the three main forms of territorial expansion (the other two being colonisation and diplomatic annexation), but it is in war that the most visible changes in power are made.


Casus Belli

The main causes of war are over the reputation of a nation or under a Casus Belli (CB). The advantage of declaring war with a Casus belli is that it usually reduces the stability hit suffered. Without a CB Reputation penalties can be accrued, often leading to other nations invading as well. A CB can also provide other bonuses, such as reduced Reputation and War score penalties. The option to select a specific Casus Belli was introduced in Heir to the Throne. This greatly expanded the variety of reasons for war, though limiting nations to fighting under one at a time.

Call to Arms

Wars can be joined by additional nations via a Call to Arms, mainly coming from alliances and guarantees, among others (see more on the main article). Any nation turning down a call will immediately lose its alliance, however, and suffer an additional penalty to its relations with the ally.

Whether or not an AI nation will answer a Call to Arms is largely determined by the level of trust between the two nations. The more trustworthy the Caller is deemed, the more likely the AI is to assist it. If the AI is already engaged in a war of its own, it may also be less likely to accept and be involved on too many fronts.

In Divine Wind (DW.png) a nation may choose not to call its allies into a war at the outset, and retain the option to summon them later without the cost of a diplomat.

Diplomacy during War

While war is being fought, normal diplomatic options that would be available, such as Guarantees and Gifts, cannot be taken by either participant. The only two available options are suing for peace and issuing embargoes. Also available in war time, embargoes prohibit additional merchants from being sent to the embargoer's Centres of Trade, though merchants already in place will remain until competed away.

Military Access

War is frequently fought with at least one nation between the two at war. If this nation is not allied or actively participating, one nation or both will attempt to gain Military access from it to guarantee safe passage. If the Holy Roman Emperor is one of the participants nations may cross over Imperial territory without first acquiring military access; though if they do not have access they may be stuck when the war ends.


Players should note that, while in a war, any army maintenance they have lowered should be raised again, lest morale suffer as a result.

War taxes

Wars are often funded through the raising of War taxes. There are, however, two sides to this. What may appear as the easiest way to raise capital for an invasion often results in its collapse for new players. Unaware that repeated war taxes raise War Exhaustion and increase the chances of Rebels, beginners will often invade small neighbours with the sole intent of taxing their citizens while obliterating the other nation. As Inflation and maintenance costs destroy his economy, the new player will often wonder what went wrong.


War subsidies are another, less reliable source of wartime income. They can only be determined by the giving nation, and therefore not worth depending on. They will most often be given if the nation with whom the player is fighting is a rival of another.


The end of a war is often a good place to earn funds, however. For colonial powers in the Americas, rather than demanding provinces they often instead seize all their foe's ducats in a series of brutal invasions. This can result in the invader becoming the richest nation in the region, fast outpacing her neighbours in the same Technology group.


While in a state of war, participants accrue War Exhaustion, a penalty that increases national Revolt risk and lowers army morale.

Similar to war exhaustion, War Capacity is a measure of how capable a nation is of continuing a war, ranging from a low of 0 to a high of 100. AI nations will generally take this into account when signing for peace - if they have too low a War capacity, they will be deemed incapable of successfully fighting the war. It is calculated with such factors as relative army size, provinces captured, manpower, war exhaustion, and numerous other factors. Unlike War exhaustion, however, this value does not actually effect a nation's economy; it is purely diplomatic.

War score, the measure of how well a war is going, is shown ranging from -100% (where the player or aggressor is losing utterly), to 0% (where nothing has happened, or the war is perfectly even), to 100% (where the defender or player's land has been entirely taken). It increases or decreases based on the relative value of provinces taken, ports blockaded, and battles fought.

Battles & sieges

Battles are the most common action in a war, and occur inevitably when two opposing armies or navies share a province. The outcome of a battle is decided on factors such as terrain, generals or admirals, and army size, among others. Massive numbers of troops can be lost against a smaller foe, simply by attacking across a river, for example. However, even a large engagement, in which one side's military may be entirely crushed, does not overly change the war score. A battle is rarely more than a fraction of a percent, but the act of defeating an enemy army has effects on morale and War capacity as well.

Though battles are often unavoidable, the main damage is done via siege. The brutality of an assault when opportune is not to be underestimated; the capital or a wealthy province can tip the balance in one nation's favour. While any form of land troop can conduct a siege, only infantry and artillery have specific functions. Artillery units increase the likelihood of a breach occurring in a fort's walls and, depending on their numbers, increase the speed of a siege, while infantry are the troops that launch the assault. Cavalry merely serve to encircle the city without participating in any fighting with the defenders.


Colonies tend to be a relatively minor part of warfare in the early game, but when nations begin to adopt the National Idea Quest for the New World colonisation becomes a way of protecting sovereignty and seizing wealth. Historically colonies were often a source of contention; this is reflected by the colonialism Casus Belli which permits the capture of enemy colonies near the player's.

Armies have the Seize colony option available to them while in enemy colonies; this option accrues the player a quantity of infamy, and gives them the colony. This is only available when the province's population is under 1000. Once seized, a colony no longer counts towards war score unless it is reoccupied.


Wars will always end with at least one dissatisfied participant. They end either through a negotiated peace or from general inactivity - i.e. if a certain time elapses without conflict the war is immediately ended with a Status Quo. Frequently an artificial ceasefire is created, when both sides are evenly matched: a White peace. Once peace has been signed, a truce follows and lasts for five (ten DW.png) years. Breaking this truce results in a stability hit for the attacker, creating an incentive to stay neutral.

A nation can, while negotiating the peace, either offer a tribute, demand a tribute, Demand Annexation or request a White Peace. Depending on the war score accumulated, as well as other factors such as war exhaustion, a nation may be more or less likely to accept this.

Demand Tribute

Demanding a tribute is generally the offer proposed by the war's victor. It enables the nation to demand its foe do one or more of the following:

In some special cases more options are made available to the victor:

  • Convert to the victor's religion (if they are of different faiths)
  • Enter a personal union (available in succession wars)

If the tribute demand is significantly better than what is expected based on warscore, then the other party will lose stability if it is refused. In rare cases, there may be a sweet spot where the foreign country refuses to pay tribute yet it's better than what should be expected.

Offer Tribute

This option is not necessarily only for the nation losing the war. When fighting multiple wars it can often be wise to back out of one quickly, paying a tribute to ensure that the war is brought to a close. There are a number of different options available when offering a tribute, such as:

  • Paying tribute in ducats
  • Surrendering provinces
  • Ceding cores on others' territories
  • Annul treaties


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


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