If you feel powerful or you are in a good relationship you can demand or ask them for something. If you make such offer it will, most of the times, be accepted. If you have good relations with other civilizations you can receive a message about a trade or scientific cooperation.

If you don't fulfill them it doesn't necessary mean that a war is nigh, even if they usually say that, but many of those civilizations will tell you about their attitude towards you. If other civilization see you as a weaker one you will often receive a scornful information or demands. Usually, other civilizations don't like when you colonize a system that is close to their borders or when you establish an outpost in their system. The option for negotiations becomes available when you meet a fleet or fly to a system that belongs to other civilization. If you want to have more options then you should develop a few technologies from Empire Development tree. Diplomacy can quickly become an important element that allows you to be in good relations with other civilizations.

Endless space 2 pirate diplomacy