Once your galactic borders bump into rival empires, you will find that expansion is only really possible through warfare. War in Stellaris has both a space combat component and a land combat component. You can read about those aspects in their respective articles. This article will deal with the diplomatic and domestic parts of war as opposed to the actual military aspect.
- 1 Declaring War
- 2 Wargoals
- 3 Warscore
- 4 The Home Front and War Policy
- 5 Peace Negotiations
- 6 Ramifications of this System
- 7 References
Unlike other Paradox grand strategy games, a reason or justification for war is not needed. An empire can declare war on any other empire if
- there is not an alliance, vassal, or tributary relationship in effect and
- there is no non-aggression treaty or running armistice (10 Years after the last war) in effect.
Selecting proper Wargoals
When starting a war, an empire should carefully consider and select strategically valuable war goals that that are militarily achievable and feasible within the possible Warscore. For example, in a war against a large Federation of which only one small member is accessible to attack, it is unlikely that the Warscore needed to force their surrender is achievable. This will make it very difficult to even negotiate a white peace, as the enemy must feel at least some pressure, in the form of accumulated Warscore, to agree to end the war.
An empire must have at least one Wargoal to declare war, either for itself or for an ally that is involved. Sometimes it is best to not press demands because of the diplomatic repercussions among other empires than may join together in response.
Peace Negotiations and Ending a war
There are a few ways to end a war. Which of the Wargoals have been achieved depends on the success of the war thus far.
White Peace: Both sides can agree to just end the war (often because another Crisis requires their attention) with neither side achieving any Wargoals.
Peace Negotiations: If the war is going at least 20% in Favor of one side, then the winning side might offer a peace claiming fewer than all of the Wargoals, or the losing side might offer to concede some Wargoals worth more than their negative Warscore to buy peace. Demands can be forced by a winning side that presses demands worth less than the current Warscore. The AI only concedes to forced demands, while a player can accept or refuse any offer, and can override Forced Demands with 100 Influence per case.
Surrender: If Warscore reaches -100% for either side or the situation becomes grim enough the AI might just surrender on its own initiative. All Wargoals are conceded to the other side in this case. In effect, this automatically forces the demands for one side.
Country loss: If a country is part of another war and ceases to exist as result of that war, the war ends without anything being given to the other side.
With 1.2 and 1.3, Alliance and Federation mechanics were combined into a single Federation Mechanic, with a Federation requiring a minimum of 2 empires.
The Federation is the more serious brother of the Defensive pact and inviting Attackers mechanic. It must include at least 2 Empires. While it does offer a unique amount of defensive cooperation, builds trust and allows military cooperation using the Federation Fleet, a conquest strategy can be made very difficult by the need for unanimous consent of the members to declare war and each member's expectation of a "fair share" of spoils from each war. Mechanically, every member of the Federation is automatically invited to any war that any member to declare. One exception to this is wars of liberation or to a lesser extent vassalization: usually AI will agree to a Liberation War even if all the Wargoals are assigned to the player. The resulting splinter empire could then be vassalized and later integrated by the player. In case an alliance member is attacked, all allies are automatically drawn into that war as defenders without need for a further declaration of war. In this case, the member being attacked has a free hand in choosing Federation Wargoals, which might be all in its own favor.
When a Federation is involved, "Federation Association Status" replaces and functions as a Non-Aggression Pact with all Federation members. The AI is very unlikely to cancel Association Status once trust has been built on both sides. This can usually be broken by Rivaling and Insulting the non-alliance party, until it cancels the agreement on it's own. As such care should be taken with future conquests.
Each member of a Federation automatically assigns 20% of its Fleet Cap to the Federation Fleet. The Federation Fleet is designed, built and commanded exclusively by the Federation President. While it can not exceed its cap, it also costs no maintenance and may be built using any of the technologies known to Federation members. This makes it a very cost-effective element of Federal defenses.
You can declare war on enemy empires for a number of wargoals, each costing different amounts of Warscore during setup and to demand in peace talks. In order to claim a wargoal the enemy has to either surrender (automatically at -100% Warscore) or they have to be selected during peacetalks. Warscore costs usually depend on the size of both the attacker and defender (the larger the cheaper are planet wargoals, and the more expensive scaling empire ones).
There are 2 basic types of Wargoals: Empire wide and Planet specific ones.
These Wargoals affect a whole Empire, rather the single planets. They are all centered around Diplomacy and internal Policy selections. Usual cooldowns and rules for undoing either kind of change apply. The specific options are:
- (50) Liberate (Empire): Target empire is forced to release the subject empire.
- (10) Humiliate: The target empire acquires a -10% happiness and -33% Influence gain for 10 years, while the attacking empire get's 100 Influence as a lump sum.
- (10) Open Borders: Only select-able if borders are closed. Forces the borders to be opened for 10 years. Does not override closed borders as part of Rivalry.
- (10) Stop Atrocities: Forces the target empires Slavery and Purging policies to be set to "Prohibited".
- (10) Artificial Intelligence Ban: Forces the target to set the Artificial Intelligence policy to "Outlawed". May only be available for Spiritualist Empires or ones that have set that themselves.
- (var) Vassalise: Turns the target empire into a vassal of the attacker. As of 1.1 scales with target size, making it impossible to vassalise big Empires. With lower target tech level this turns into making them a Protectorate. Requires unrestricted warfare policy.
- (var) Make Tributary: Turns the target empire into a Tributary. Costs slightly less the Vassalisation and does not include any obligation to defend the empire. Requires unrestricted warfare policy.
Planet Specific Wargoals
These wargoals must be chosen on a per Planet basis. Each planet in a system must be chosen separately.
Transfers the planet to the attacking empire. This option is rather expensive and has to be chosen for each planet in the System separately. It is only available if at least one of the following holds:
- Attacker has unrestricted warfare policy.
- Attacker is original owner of planet.
- Attacker is majority species on planet.
- Defender is awakened Fallen Empire.
This costs less than Cede Planet and will establish the planet(s) as an independent Empire when the war ends. All planets freed in the same War will be in the same empire. The dominant species will be whichever species is most present among all planets liberated. Liberations done across multiple wars will not unify in a existing empire. Technology will be copied mostly from the original empire, granting possible access via research agreements. The new empire will take the Liberators government ethos and be friendly towards them for liberating them. This, however, does not mean that the individual POPs on the planets of the newly liberated empire will adopt the same ethos as the liberator/new empire. They will have to shift to it through ethics divergence, with all the usual penalties for unfitting policies. As of 1.1.0 Liberating can be chosen on Capital Worlds too.
This option streamlines the process of ceding a Planet and purging the population into a single wargoal that is instantly executed once the war ends. It costs the same warscore as ceding and also incurs the full purging related diplomacy Penalty for every pop purged. Purge for all affected pops must be allowed and it can not be chosen on the Capital World of an empire.
As of 1.2 it is possible to invite an empire to a joint strike against a common enemy. The choosen empire must be able to declare war on the target and must agree to the wargoals, which usually entails picking some in its favor. Federation members are forcefully invited. Liberation seems to be a usually acceptable wargoal.
If an Empire is attacked, it can choose which Wargoals to take from the enemy empire(s) within the first 90 days. If the player attacker is in a federation and initiated the war, only his/her planets seem able to be chosen as collateral Wargoals. If attacked while in a Federation or a defense pact, the attacked party can choose the Wargoals entirely in it's favor.
Warscore decides who wins and who loses a war and can be obtained in several ways. Every Wargoal requires a certain amount of Warscore in order to demand it in peace negotiations. The enemy side surrenders if Warscore reaches -100% (effectively forced demands for every goal). For example to liberate an empire requires 50 Warscore. That means your combined total war activity has to be 50% in your favour to get that empire to submit to you in negotiations. See Peace Talks above for details.
War score is obtained in four different ways:
Every battle you fight with an enemy fleet or military station gives Warscore to the winner.
Landing troops and occupying worlds is another important method for gaining Warscore. You do not need to occupy the planets to aquire or demand them, but doing so will greatly aid your Warscore (see below). Occupied planets do not give resources to the enemy that used to own them, so it's a great way to cripple their industrial output.
Similar to occupation but without the messy ground combat, a blockade will stop resources from leaving the planet and ports from being rebuilt. Thereby starving the enemy of resources. Furthermore the Orbital Bombardment can ready the planet for the ground invasion and damage pops and buildings (depending on stance).
There are some War goals that will give Warscore for each time frame they are held by one party. As of 1.3.1 no such Wargoals are in the game.
The Home Front and War Policy
Generally there are two sides in any war: The Attacker and the Defender. War is generally not popular with your Population in particular if your side is the attacker.
The Policy Warfare limits which kinds of wargoals one can choose during war declaration. It also limits which kinds of wargoals can be assigned to a empire when choosing wargoals in a combined war declaration. It also has a large impact on how likely an AI is to ally with a player, as different war philosophies may act as a penalty.
Happiness suffers drastically during wartime. Being the attacker in a war imposes a flat -10% Happiness penalty for as long as the war runs. By default the defender has no penalty. However the Pacifist/Militarist axis can modify that on a per-pop level. Pacifist Pops get bonus Happiness while at Peace, but negative while at war. This is in addition to any penalty for being the attacker and effectively means a -10 or -30% decrease in happiness. Militarists in turn get a bonus to happiness while at war. That means they suffer no penalty while being in a offensive war and even get happiness while in a defensive war. However either of those two are population modifiers. So Ethics divergence will play a relevant role here. In any case a war will drastically increase Faction activity in an empire.
At any time during the war, you may click on the war status icon to the lower right and open negotiations. The icon opens a war status pop-up that shows how the Warscore has been calculated. At the bottom of the menu is a tab for negotiations. You will be shown both sides' Wargoals and can choose among them by clicking on the goals you want to accept as the terms of the peace, or select no goals if you want a White Peace (an agreement that no one should lose anything). You can see if the enemy AI is likely to accept the peace offer in the center of the menu. A green check mark means the AI will accept, a red X means that the AI will not.
You can only choose Wargoals from one side of the menu - you cannot use a war to swap systems from both sides. Wars must have winners and losers.
Ramifications of this System
The Wargoal & Warscore thematic is a huge part of how the Conquest and even Combat parts of the game play out.
No single Victory
The 100 Warscore cap on Wargoals and the scaling of wargoal costs per planet based on attacker and defender ensures that no matter how well you do in the fighting, you cannot take over a large empire in a single war.
About 3 planets will be the maximum you can take in one war as Wargoals. When you are a member of an alliance, conquest is not usually very effective because the Wargoals for any war must be pleasing for every one of your allies. You will also have to deal with the population of any planets that were ceded to you in the war, which will typically be unhappy about your rule and likely not share your ethos. This makes it conquest unsuitable for any empire that cannot tolerate considerable Ethics divergence.
Liberation can be used to cause more damage to the enemy due to the lower per-planet Warscore cost, but at the cost of creating another independent empire. It can also be used to restore an empire that used to exist and whose people are still around on the planets.
As the new empire shares your Ethos they will very likely be friendly if you choose to Vassalise them or invite them to your alliance. Another boon is that you do not have to deal with the conquered population (happiness from Conquering and Ethos differences), but get a productive ally right away. One that might even convert the population for you if you plan to annex or integrate them later.
This is the primary way Alliances fight wars, as no AI seems to have issues with the Ethics being chosen in the player empires Favor. In 1.0 and 1.1 was necessary to give each ally one of the to be Liberated Planets as Wargoal, while the new Empire would still take your Ethos (as initiator) and take all planets. However latest as of 1.3 this behavior was fixed and Alliance will just generally accept Liberation even if in Favor of only one party.
The relatively low cost allows you to just take the empire out as foe in one blow, but only if it does not exceed a certain size. And after 10 years they can be integrated per the usual rules for a Vassal. It does not affect their Ethos. It is generally considered an acceptable goal for Alliances War declaration, even if only one benefits from this Wargoal directly.
However keep in mind that integrating a big empire will prove prohibitively expensive and they could at any time rebel against your rule if you lack the power to keep all your Vassals in check.
This allows you to quit while you are ahead but have not achieved a total victory. This can be useful if another enemy attacked while you were busy elsewhere or your population/economy is suffering because of the war.
If you can put the AI into a suitably bad position, they will surrender and thereby cede systems you have not actually conquered. This can be used to force a planet out of the AI's control when one of their planets is too heavily fortified for a ground assault. However that usually means you needs to destroy every last fleet the enemy has, including small raiding fleets in your own territory.
Surrenders can't be rejected. If you decide to accept all the war demands and surrender to your opponent, the war immediately ends. The same is true for AI surrenders.
Without modding, there is no way to prevent AI surrenders. If you want to cede a planet, but that planet is already captured by another empire, it's impossible for you to demand this planet to offer peace. However, actually they CAN surrender you for the impossible war demand, and if they do, the captured planet will be yours, and all the armies are force out. The same is true for liberating planets. If all of the attacker's war demands are impossible to surrender for, the human defender can't surrender at all, however, an AI defender CAN surrender for no demands, and the attacker won't get anything.
After the war is over - no matter how it ends - an automatic mutual Non-Aggression pact lasting 10 years is signed. This means that for 10 years no further hostilities can occur between the two parties. So even if you only achieved a partial success, the other Empire is effectively removed as a problem for the next 10 years. However keep in mind that a 3rd party might swoop in during this Armistice, taking out the now weakened foe. Also they might choose to join an alliance in the interim. In that way, any decisively won war could end up making another enemy (alliance) stronger in the short to mid term.
It used to be possible to declare a war without the intent of fighting in order to get the Militarist Bonus to happiness, but with the rework of the bonuses, that no longer works. However, Pacifists do get a happiness malus for even Defensive wars, so declaring a war on them will damage them while also locking them out of diplomatic agreements.