Even if you're the type of emperor who shoots first and never talks, you're still going to receive communications from the leaders of the galaxy's other empires. Before you invade a particular race's space, you might consider reviewing what you know about them. Most of the time, it's prudent to avoid being at war with more than one other empire at a time (or two if you're ambitious). If you're not a slavering warlord, the art of diplomacy is even more important.
Overview[ | ]
All your diplomatic efforts are conducted via the Race Relations screen which contains a summary of your relations and allows you to ask for an audience (a meeting, essentially). Diplomacy begins whenever your ships become capable of reaching one of a race's colonies or theirs can reach yours.
When you first meet a new race, the flavour of the diplomatic contact depends entirely on their emperor's personality. Most leaders act tough at first, even (or perhaps especially) when they perceive themselves to be negotiating from a position of weakness.
How you react, what you do during your usual activities, and what you ask or offer during audiences determines the ongoing status of your relations. This status is measured on a sliding scale (the one next to the portrait of the other empire's ambassador). It's a linear scale, where your deeds and misdeeds are totaled to produce a summary.
Audiences[ | ]
One thing to keep in mind is that other emperors are just as busy as you are. Repeated audiences encourage impatience, and even the most friendly ally eventually refuses to meet with you. If this happens, wait a few turns. Time restores your opponent's willingness to communicate.
Reputation[ | ]
For the most part, the diplomatic effects of your actions are exactly what you'd expect. Attacking a colony has a negative effect, for example, and a gift of technology a positive one. There are, however, a few things you can do during an audience that warrant explanation.