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 Ending wars
- 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 no relationship that prevents war (vassal, non-aggression treaty, guarantee of independence, or defensive pact), and
- there is no running armistice (10-year truce after ending a war or a relationship that prevents war)
Note: as of Version 1.6, an empire is no longer prevented from declaring war against tributaries, either their own or of another star empire. There are however limitations to the war goals when declaring war on the empire's own tributary.
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.
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 its 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 gets 100 Influence as a lump sum.
- (10) Open Borders: Only selectable if borders are closed. Forces the borders to be opened for 10 years. Does not override closed borders as part of Rivalry.
- (20) Stop Atrocities: Forces the target empire's 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 an 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.
The chosen 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 always invited, but will vote on whether or not to go to war.
Fellow empires cannot be invited if the one declaring the war is a member of a Federation, unless they are also members of the Federation.
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 its favor.
This section may contain outdated information that is inaccurate for the current version of the game. The last version it was verified as up to date for was 1.5.
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 below. In v1.5 Banks, the amount of warscore attained for has been adjusted such that occupying the planets specifically stated in the war goals give vastly increased warscore relative to other activities such as space battles and blockades, which now give very little. In other words, having boots on the ground is the primary/only way to achieve to force the hostile party to surrender.
All battles fought with an enemy fleet or military station gives Warscore to the winner. In the late-game, this reward can be as low as 0.1%.
A blockade occurs when a hostile empire stations a fleet in the orbit of the planet. This stops all resource flow and ports being rebuilt, effectively rendering the planet useless to the rest of the empire. Orbital bombardment will also occur during blockade - this targets enemy fortification, with collateral damage potentially killing pops and damaging/destroying buildings. It is usually necessary to reduce the fortification value to near-zero so as to render the planet ripe for invasion.
Landing troops and occupying worlds is the primary way to gain Warscore. Any occupied planet will grant warscore regardless of whether they have been included in the initial demands, but those are part of the demands give more warscore while occupied.
An occupied planet grants no resources to its host empire, since all government activities are be suspended and the citizens are unemployed. Due to this, slaves and populations undergoing purging will become very happy while citizens will become unhappy.
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 an 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.
War can also affect pop happiness in an empire. As of Version 1.6, these effects depend on the political factions of the citizens, and no longer directly affect the population themselves as in earlier versions. Generally, aggresive wars please militarist factions, and upsets pacifist factions, while keeping longterm peace (10+ years) has the opposite effect. The detailed effects is shown in the individual faction details in the Factions tab. War can also have an effect on the ethic divergence/convergence of your empire. This effect can be very complicated. Fighting a defensive war against Xenos gives a large boost to Xenophobia attraction (and thus gives a penalty on Xenophilia attraction), fighting an aggresive war boosts militarist attraction, and the possible unrest resulted from occupations and faction dissatisfaction could randomly boost attraction for Spiritualist, Materialist, Pacifist, or Militarist ethics on specific planets. Hive minds are not affected by factions or happiness, thus they take no reward or penality in this aspect.
With 100% warscore, you can force your opponent to surrender immediately. This is instantly achieved when all enemy planets are occupied by you or your allies, even if your side had lost all but one planet and every major fleet battle.
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.
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. However, you can select no goals from either side, proposing a White Peace that would end the war as a tie.
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. AI empires decide based on the following factors:
|All demands met||+1000 (yes)||Offering a complete surrender|
|Utterly defeated||+1000 (yes)||100% warscore|
|Not enough warscore||-1000 (no)||<20% warscore and making any demands|
|Warscore||-100 to +99|
|Peace offer||-100 to +99||Total warscore-cost of demands or concessions (0 for white peace)|
|Length of war||-48 to +?|| More willing to accept any peace proposal if the war has been long|
((Duration of war in months) - 120), multiplied by 0.4 if negative or 0.2 if positive (?)
|Relative navy strength||-? to +20|
|Would be annexed||-30||Less willing to surrender when asked to cede all of the empire's planets|
|Would likely be purged||-30||Less willing to surrender to fanatical purifiers|
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.
If an empire ceases to exist, the war ends without anything being given to the other side. This can happen as a result of them losing a second, simultaneous war.
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 against large empires
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. Both traditions and technologies are available to reduce warscore demands as of v1.5 - with traditions boosting specific types of conquest goals and two society technologies boosting all types of conquest goals (except for Humiliate, which always cost 10 warscore).
At the start of the game, it will be difficult to take more than 3~4 planets in a single war unless the player chooses the supremacy tradition Right of Conquest. This only requires two traditions - one to open the tree and one to pip the conquest tradition, - and so can be done quickly as an aggressive opening.
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.
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. The domination tree's opening bonus decreases the cost of vassalisation, thus effectively increasing the maximum size. 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, or too far for your FTL to reach. However that usually means you needs to occupy more enemy planets than you otherwise would have.
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 truce lasting 10 years is signed. This means that for 10 years no further direct 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 a federation in the interim. In that way, any decisively won war could end up making another enemy stronger in the short to mid term.
However, there is a way to attack an empire before the truce expires. If the target empire is guaranteeing the dependence or in a defensive pact with another empire that does not have a truce with you, you can declare war on that empire, and can still select most war goals involving the target empire. The same method works if the target empire is in a federation, however, this usually means that this empire would have to enter the federation or defensive pact after the end of the first war, otherwise the other empire(s) would have taken part in the first war, and thus share the same truce. This can also be used to attack a non-federation ally, such as the empire that you are currently in a defensive pact or non aggression pact with, or the empire that is guaranteeing your independence, or whose independence is guaranteed by you. This however only works when you are at peace, and would instantly cancel your alliance with eachother.
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, Pacifist factions 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.
It is now possible to attract your population towards Militarist ethic by declaring an aggressive war without actually fighting, but in order to cheer up Militarist factions one would have to enforce aggressive wargoals.