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This article is for the PC version of Stellaris only.

As long as there is at least one empire present that is not a Fanatic Pacifist.png Fanatic Pacifist empire, war is an almost inevitable aspect in any game and can be anything from a small border skirmish to a massive interstellar war of conquest.

Wars are how mid-game and end-game borders are usually drawn and Militarist.pngFanatic Militarist.png Militaristic empires may favor war over diplomacy.

Casus Belli and Wargoals

In order for an empire to start a war, it needs a Casus Belli – a reason to declare war. Each Casus Belli grants access to at least one type of Wargoal, which represents the purpose of the war.

Casus Belli Wargoals Requirements Surrender Acceptance Surrender Effect Status Quo Effect
Claim Diplomacy war.png Conquer Declaring empire has existing claims on the target empire's systems −10 per system
−100 per colonized system
Loser cedes all systems with Claims to the winner, even if not occupied. All systems occupied by the loser are returned, regardless of Claims. Each side keeps all systems they have (fully, not just partly) occupied if they have claims on them.
Subjugation Diplomacy isvassal.png Vassalize
Diplomacy istributary.png Make Tributary
Diplomacy istributary.png Make Subsidiary
Declaring empire used the Demand Vassal\Protectorate\Tribute\Subsidiary Status diplomatic action and the demand was refused by the target empire −100 Loser becomes a subject of the winner, along with all its vassals (if any). If the empire is part of a Federation, it will be removed from it. All systems fully occupied by the attacker will be turned into a subject of the attacker.
Animosity Diplomacy opinion.png Humiliate Declaring empire has a Diplomacy rivalry.png Rivalry against the target empire −50 Loser loses all of its Influence.png Influence, gains -33% Influence.png Monthly Influence Gain and -10% Mod pop happiness.png Happiness for 10 years. Winner gains 100 influence. None
Ideology Diplomacy force ideology.png Impose Ideology Declaring empire has its War Philosophy policy set to Liberation Wars
Neither empire is a Gestalt consciousness.png Gestalt Consciousness
−100 Loser is forced to shift ethics towards those of the winner and will leave a Federation if it's part of one. All systems fully occupied by the attacker will be turned into an empire with the attacker's ethics.
Subject Diplomacy integrating.png Independence Available to subject empires
Can only be used against the Overlord
0 If the subject wins it becomes independent but the former Overlord will gain Claims on the former subject's systems. None
Despoliation Diplomacy war.png Plunder Declaring empire has the Civic barbaric despoilers.png Barbaric Despoilers civic −25 The Loser loses 500 Energy Credits.png Energy and 500 Minerals.png Minerals for every 3 planets they own, rounded up, and capped at 60 planets, for a maximum of 10,000 Minerals.png Minerals and Energy Credits.png Energy gained. The Winner will then gain those resources even if the loser didn't have enough. Loser gets -20% reduced Energy Credits.pngMinerals.png production for 10 years. None
Expropriation Diplomacy commercial pact.png Expel Corporation Declaring empire has at least one Branch Office on its planets and the Branch Office owner (corporate) empire does not have a Commercial Pact with the declaring empire. −50 The Corporate empire closes all Branch Offices on the attacker's planets. Attacker gains 1000 Energy Credits.png Energy per Branch Office. The Corporate empire closes all Branch Offices on the attacker's planets. Corporate empire gains 1000 Energy Credits.png Energy per Branch Office.
Hostile Takeover Diplomacy commercial pact.png Hostile Takeover Both empires have Auth corporate.png Corporate authority
Target empire has at least one Branch Office on any planet
−75 All Branch Offices are transferred to the victor. If the target empire is part of a Federation then any Branch Offices present on planets owned by federation members are kept. None
Colossus Diplomatic rival mutual icon.png Colossus Declaring empire has a Colossus and a Diplomacy rivalry.png Rivalry against the target empire −1000 (Total War) If the attacker's Colossus is destroyed total war ends and the war reverts to a normal war. None
Stop Colossus Diplomatic rival mutual icon.png Stop Colossus Target empire has a Colossus −1000 (Total War) If the defender's Colossus is destroyed total war ends and the war reverts to a normal war. None
Purification Diplomacy war.png Cleansing Declaring empire has the Civic fanatic purifiers.png Fanatic Purifiers or Civic machine terminator.png Determined Exterminator civic −1000 (Total War) The defeated empire is annexed completely
AI empires will never surrender to this war goal
Hunger Diplomacy war.png Absorption Declaring empire has the Civic devouring swarm.png Devouring Swarm
Sublimation Diplomacy war.png Assimilation Declaring empire has the Civic machine assimilator.png Driven Assimilator
Containment Diplomacy war.png End Threat Target empire is Civic fanatic purifiers.pngCivic devouring swarm.pngCivic machine terminator.pngCivic machine assimilator.png genocidal or an Awakened Empire
Punishment Diplomacy opinion.png Humiliate
Diplomacy opinion.png Stop Atrocities
Diplomacy opinion.png Outlaw AI
Diplomacy opinion.png Cleanse Holy Worlds
Diplomacy opinion.png Decontaminate Border Territories
Only available to Fallen Empires with low opinion
Either the Fallen Empire's demand was refused or they were rivaled
0 The refused demands are enforced.
Defeated empire is humiliated and loses all of its Influence.png Influence, gains -33% Influence.png Monthly Influence Gain and -10% Mod pop happiness.png Happiness for 10 years.
Defeated empire's leaders will be executed if not immortal.
Claim the Galatron Diplomacy closed borders.png Take Galatron Target empire has the Galatron Relic −40 Winner keeps (or acquires) the Galatron Relic and gains 100 Influence.png Influence. Loser loses all of its Influence, gains -33% Influence.png Monthly Influence Gain and -10% Mod pop happiness.png Happiness for 10 years. None
Event War in Heaven
Machine Uprising
Available to Fallen Empires and AI Rebellions via a Special Event
Can only be used against a designated target empire.
−1000 (Total War) No.png None

Once a war is declared, the targeted empire has a year to choose its Wargoals. However, certain wargoals will automatically cause the target to issue certain counter wargoals if chosen (As an example; The "Independence" wargoal will automatically cause the targeted empire to issue the "Remain Overlord" wargoal).

Casus Belli with −1000 surrender acceptance lead to a Total War. Total Wars do not require claims and occupied systems automatically change ownership during a Total War. When a system is conquered, it will be occupied by the participant with the highest claim count on it, otherwise, it will be occupied by the conqueror if none of their allies claim the system. If an empire loses all its colonized systems during a Total War it will be destroyed and all of its Starbases in uncolonized systems will be removed, making the territory neutral again.


While Casus Belli and Wargoals represent the purpose of a war, claims represent territorial ambitions—the systems which will be controlled by the victor. Which systems can be claimed depends on an empire's War Philosophy policy, with the Unrestricted Warfare philosophy allowing for the claiming of any system not owned by a fellow Federation member. It is possible to claim the same system multiple times to gain a stronger claim on it, which can outbid competing claims of allies when going to war together. If multiple empires have equal-strength claims on a system, the oldest claim will be considered the strongest. In general, claims are expensive in the early game and become less so later on to allow for more decisive wars to be fought. Claims cannot be rescinded while at war.

Claims cost Influence.png influence, the base cost being 50, modified by the following:

  • +25 per hyperlane distance from owned territory
  • +25 if the system has at least a lvl 1 Starbase (i.e. not just an Outpost)
  • +25 if the system is colonized
  • +100% during offensive war
  • –10% Agenda Expansionist Overtures agenda
  • –10% Tech interstellar campaigns.png Interstellar Campaigns technology
  • –10% Tech galactic campaigns.png Galactic Campaigns technology
  • –10% Civic nationalistic zeal.png Nationalistic Zeal civic
  • –10% Militarist.png Militarist
  • –20% Fanatic Militarist.png Fanatic Militarist
  • –20% Ap interstellar dominion.png Interstellar Dominion ascension perk
  • –20% Diplomacy rivalry.png Rivalry (Only on systems owned by the rival empire)

During defensive wars, claims can be made for any system by the defender, regardless of the War Philosophy policy. Any system lost by any side during a conquest war or total war will give the former owner a claim on it.

Winning a war with any non-conquest wargoals will always enforce all claims made by the empires on the winning first, regardless of whether these systems were occupied. So for example, winning war with a subjugation and conquest wargoals against other empire(s) will make it (and any of its allies) lose all systems that have claims on them, including ones made by the now-overlord empire, and only then become a vassal, with territory reduced like that and free claims of strength 10 on all lost systems.

Federation Warfare

The Federation is the more serious counterpart of the Defensive pact and the inviting attackers mechanic. While it does offer a unique amount of defensive cooperation, builds trust and allows military cooperation using the federation fleet, establishing a conquest strategy can be made very difficult because of 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. The empire that wishes to declare war can ask the other federation members for their war goals by using the "Suggest Demands" button. However, AI empires will be very hesitant to declare war on empires that are much stronger than them or have very powerful allies. In that case, the "Suggest Demands" button will not give any results.

One exception to this are wars of liberation or to a lesser extent vassalization: usually AI will agree to a liberation war even if all the war goals are assigned to the player. The resulting splinter empire could then be vassalized and later integrated by the player.

If a federation member is attacked, all allies automatically join the 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 war goals, which can freely be wholly in its own favor.

For empires that are not members of a given federation, Federation Association Status functions as a Non-Aggression Pact with all members of the respective federation. The AI is very unlikely to cancel Association Status once trust has been built on both sides. This can usually be broken by rivaling or insulting the non-member party, until it cancels the agreement on its own. Therefore, care should be taken with granting Association Status to empires that the player may want to attack in the future.

War Exhaustion

War Exhaustion goes from 0% to 100%, and measures the total weariness and attrition suffered by all empires on one side in a war. War Exhaustion goes up from suffering losses during Space and Ground Warfare, destruction of planets (either from Colossus weapons or Bombardment armageddon.png Armageddon Bombardment), and a passive accumulation over time called Attrition. When a side's War Exhaustion hits 100% they can be forced into a Status Quo peace after 24 months. The speed at which War Exhaustion accumulates is influenced by factors such as ethics, traditions, technology and the amount of claims being pressed - an empire that is fighting to hold onto a handful of border systems will tire faster in a a costly conflict than one whose very independence is being threatened. War Exhaustion also means that an Empire that is losing a war can still fight to minimize their territorial losses by inflicting high War Exhaustion on the enemy.

War Exhaust Gain can be reduced by the following:

Source Reduction
Civic media conglomerate.png Media Conglomerate civic -5%
Civic nationalistic zeal.png Nationalistic Zeal civic -10%
Tech interstellar campaigns.png Interstellar Campaigns technology -10%
Tech galactic campaigns.png Galactic Campaigns technology -10%
Gestalt consciousness.png Gestalt Consciousness -20%

Ending Wars

Wars can end in two ways: With the surrender of either side or with a negotiated Status Quo peace.

Surrender means that the victor's Wargoal is enforced, and any claims the winning side has on the losing side are automatically ceded regardless of occupation status. All wars except those of independence have a negative surrender acceptance, which are countered by factors such as Relative Navy Strength (up to +50), War Exhaustion (up to +100) and Occupation Percentage (up to +100). If an empire and all of its allies (if any) has lost control of all of its systems and/or planets during the war, then a surrender will occur automatically.

Status Quo means that the war has reached a point where neither side is able to score a decisive victory against the other or all wargoals have been achieved decisively before any major battles took place, and both sides agree to cease hostilities and settle for whatever gains or losses they have acquired/suffered. Under a Status Quo peace, all fully occupied systems claimed by a belligerent empire are ceded to the belligerent with the strongest Claim. In the case of a tie, whoever has the oldest claim on the system is considered the stronger claimant. Since AI won't surrender to a Total War, they can end only in Status Quo unless the opposing empire is destroyed wholesale. Status Quo can be enforced by any side as soon as 24 months passed since the opposing side reached 100% War Exhaustion and if both sides reach 100% War Exhaustion the Status Quo is enforced automatically after 24 months.

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