When commanders meet on the planet they can start to engage in diplomacy. Diplomacy allows players to make treaties and pacts, trade resources or declare war. Most diplomacy mechanics are disabled in games with teams enabled.
The diplomatic state represents the degree of mutual cooperation between two commanders. Diplomatic states further affect opinions, race relations and reputation. As the diplomatic state between two commanders increases, they will have an increased base opinion and be able form better treaties.
The reputation of a player is affected by all diplomatic states with others. Having better diplomatic states increases reputation, where having wars or breaking a diplomatic pact lowers reputation.
Vassalage is a locked diplomatic state that can be offered or asked for during war. A commander in a vassalage state has limited diplomatic options:
- Only the overlord can end the vassalage by declaring war
- Only the overlord can call the vassal to war and the vassal is forced to accept
- Vassal commanders can no longer denounce or warn other commanders
- Treaties cannot be broken by the vassal
- The vassal is freed if the overlord is defeated
Casus belli, or reasons for war, represent a point value that are used to denounce commanders and declare war on them. Declaring war without a causus belli reduces popular support while using a casus belli increases it depending on the amount of casus belli. Casus belli are obtained when a commanders is actively provoked by another commander, through the 24px Fabricate Casus Belli covert operation or automatically by having a low reputation or race relation. Casus belli can be resolved through either war or negociation.
Automatic casusbelli are received by having a low reputation or race relation with the race of the opposing player. They can not be forgiven through negotiation but can be used in war declarations.
Popular support represents how supportive a commander's population is when they lead their faction into war. High popular support causes less unit upkeep, higher morale units and more Happiness in colonies while low popular support does the opposite.
Declaring war without any Casus Belli leads to a high popular support penalty while the war lasts. Declaring war with a casus belli grants popular support for a limited time. When a war ends the popular support gained or lost during that war is reset.
Each casus belli held towards a commander reduces opinion by 50. If a casus belli are removed via negociation the grudge opinion generated by that casus belli will be reduced by 20 each turn.