Diplomacy is the fine art of managing and taking advantage of relations with other factions. Various diplomatic modifiers affect relations, depending on the personality of the other faction. The mood of a faction towards another determines which diplomatic actions are available and how the AI will behave.
The relationship between two factions is represented by a score that ranges from -10 (hates) to 10 (loves). A value can exceed that range, but it will have no effect on gameplay.
There are no perfectly rational beings in the galaxy and cooperation must be earned. Factors determining how one faction relates to another depend on their respective personalities and include their similarity, mutual history, relative strength, treatment of other factions, active treaties, and more. See diplomatic modifiers for a complete list.
Diplomatic actions include declaring war, giving or requesting gifts, and negotiating treaties and trades. The available actions and treaties depend on their relations, how long two factions have known each other and the researched diplomatic technologies.