As long as there is at least one empire present that is not Fanatic Pacifist, 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.
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. Once a war is declared, the attacked empire has a year to choose its wargoals.
The following wargoals are paired together. If an empire starts a war with either of the paired wargoals the defender will automatically use the other wargoal and cannot pick another one.
Paired wargoals with −1000 surrender acceptance lead to a Total War. In Total Wars, all occupied systems automatically change ownership. Their new owner will be the participant with the highest claim count on it or the occupier if none of his allies claim this system.
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 the competing claims of allies when going to war together. If multiple empires have equal-strength claims on a system the oldest claim is 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 Claims cannot be rescinded while at war.
- +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% Expansionist Overtures agenda
- –10% Belligerent or Supremacist policy
- –10% Interstellar Campaigns technology
- –10% Galactic Campaigns technology
- –10% Nationalistic Zeal civic
- –10% Martial Alliance level 2 perk
- –10% Militarist
- –20% Fanatic Militarist
- –20% Interstellar Dominion ascension perk
- –20% Great Khan's Throne relic
- –20% Rivalry (multiplicative, after other multipliers)
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 war with any non-conquest wargoals will always enforce all claims made by the winning side empires first, regardless of whether these systems were occupied. So for example, winning war with subjugation wargoal 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.
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 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.
In case 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 might be all in its own favor.
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 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 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:
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; any claims the winning side has on the losing side are automatically ceded regardless of occupation status and the defeated empire is forced to keep borders open towards the attacker for 10 years. 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. In addition both sides must keep their borders open towards each other for 10 years. Since AI won't surrender to a Total War, they can end only in Status Quo or the destruction of one empire. Status Quo can be enforced by either side as soon as 24 months passed since the opposing side reach 100% war exhaustion, and if both sides reach 100% war exhaustion, the Status Quo is enforced automatically after 24 months.
Wars will always be named after the attacker and the defender, followed by one of the following names depending on the wargoal: