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 (CB for short), or reasons for war, represent a point value that are used to denounce commanders and declare war on them. Declaring war without a CB reduces popular support while using a CB increases it depending on the amount of CB. CBs are obtained when a commanders is actively provoked by another commander, through the Fabricate Casus belli covert operation or automatically by having a low reputation or race relation. CB can be resolved through either war or negotiation.
Automatic CBs 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 CB leads to a high popular support penalty while the war lasts. Declaring war with a CB grants popular support for a limited time. When a war ends the popular support gained or lost during that war is reset.
Each CB held towards a commander reduces opinion by 50. If a CB are removed via negotiation the grudge opinion generated by that CB will be reduced by 20 each turn.