Diplomacy in Stellaris draws on the model of previous Paradox games. Each AI nation has a "relation" value which is increased by friendly actions, diplomatic agreements, and similar ethos - and decreased by hostile behavior and opposing or hostile ethos. The basic diplomatic actions are as follows; most of hese actions cost the influence resource to carry out.
- Establishing an embassy to improve relations with an AI nation
- Sending an insult to reduce relations
- Offering an alliance (a strong alliance with 4 or more participants may become a Federation_
- Offering a trade agreement
- Demanding a weaker AI enemy becomes your vassal
- Declaring war
- Declaring another nation your rival (resulting in both parties gaining Influence)
In most circumstances, the AI's diplomacy - both in response to the player's offers, and the actions the AI initiates - is determined by the AI nation's relations with the player nations, the AI nation's own ethics, and also by the threat level the AI perceives from the player and from other AI nations.
Wars are governed by a war goals / war score system. When declaring war a nation selects war goals which they are trying to achieve. (If attacking as part of an alliance, the whole alliance has to agree the goals, including who gets which planets in the event of victory). Success in space combat and occupying or blockading enemy worlds increases your war score and the higher your war score and the worse your enemy's assessment of their own position, the more you can demand in a peace treaty. Naturally, if you force a harsh peace settlement on your opponent, you will make other nations around you feel threatened, making their reactions worse.