Civ/Sid Meier's Alpha Centauri/Diplomacy

Negotiating with factions.
Negotiation with other factions is allowed once you have their commlink frequency, which means you must encounter them physically on the map (you units need to be in adjected squares), receive it from a faction you already have contact with, discover it in a Unity pod, or receive it through the Empath Guild Secret Project. Negotiating with other factions is very important, as it allows you to exchange technologies, maps and commlink frequencies, enter into formal treaties and pacts, declare war on one another, and synchronise your attacks on a third faction.
Formal Relationships.
Your relationship with other factions consists of one formal value and an informal one. Your formal status with other factions is one of the following: Vendetta, Truce, Treaty and Pact. Additionally, you may have an "informal truce" with another faction.
"Vendetta" is rather self-explanatory: this faction leader has made a promise to destroy your faction, or at least bust it up pretty good. Unless you are the aggressor, of course. "Truce" (or "Blood Truce") means that your two factions have agreed not to attack each other, at least for the time being. "Treaty" ("Treaty of Friendship") is just that, a treaty which represents your two factions' friendship. More importantly, a Treaty means the citizens and organizations of two faction will now trade with each other, meaning you get extra energy credits, straight into the state coffers. The final kind of formal relationship is the Pact ("Pact of Brotherhood"). This means your two factions are committed to helping each other out, which has several manifestations. First, commerce rates double between your two factions, and secondly, your units may stack theirs. This means closer military cooperation is possible, you may defend each other's bases, and your pact mate might even give you units "to defend your territory against aggression".
When you first meet a faction, you have no formal relationship, or an informal truce. This means that you are not at war with this faction, but no actual agreements have been made. From this point, you may engage in war with the other faction, remain at an unease peace, or commit to a formal relationship. Whether the other faction agrees to a treaty or a pact depends on how they feel towards you, which is the next thing we're going to talk about.
Informal Relationships &amp; Reputation.
The informal relationship with another faction is simply how much the other faction leader likes you. This value ranges from "Magnanimous" (or "Submissive" if threatened) to "Seething", and it hinges on three things: how strong your faction is (the stronger the faction, the more respect other factions have for you, and the more they like you), your actions towards this faction, and your Social Engineering choices. If you have attacked this faction (or worse; are at war with it) the faction leader will like you less, whereas if you have traded or presented gifts to this faction, they will like you more.
As you might have guessed by now, a faction leader's opinion of you decides how likely it is that he or she will attack you. Another factor also plays into this, the faction leader's "personality". Some, such as Yang or Santiago, are more likely to attack you than others, such as Deidre and Aki-Zeta. This is important to keep in mind, especially in the beginning of the game, so that you can plan the training and construction of military forces accordingly. If you start next to, say, the Usurpers, you can be sure that unless you are far stronger than them, they will declare war soon, especially on higher difficulty levels, and even moreso if you have turned on the "Aggressive Factions" option. Faction leader personalities can be randomized under "Game Rules".
Your Social Engineering choices also play a part in how other faction leaders behave. Each faction has one SE choice on their agenda, and one SE choice aversion (see Understanding the Factions). If you are currently running the SE choice on the agenda of a given faction, this faction will think more highly of you, and if you are employing their aversion, they will criticise you for it, and it may even be a contributing cause for open conflict. Faction leader agendas can also be randomized.
In the course of the game, you will very likely make deals and trades with other factions, as mentioned. Whether or not the other faction leaders will accept the deal you propose depends on their opinion towards you (or "mood"), and your "reputation". Reputation starts at "Noble" and can diminish as a result of dishonourable actions; your reputation is affected by whether or not you honour formal agreements. If, for example, you have signed a Treaty of Friendship with another faction, and attack them "without" waiting for the treaty to expire, your reputation will incur a hit. The same applies to Blood Truce.
Note that while you may not attack a pact mate (your unit will only stack with his or hers), you may still incur a reputation hit if your probe teams are caught trying to operate on one of your pact mate's bases or units. If one of your probe teams are indeed compromised, the target faction has sufficient reason, with regards to reputation, to move against you. This means that if you attempt to steal technology from a pact mate and are caught, your pact mate may declare vendetta against you without going through the normal channels. The same, of course, is the case for a human player.
The Planetary Council.
The Planetary Council is a rare feature among Civ games; just like the United Nations of Civilization IV. The Planetary Council consists of every human faction in the game (not counting those who have been eliminated or committed major atrocities). When a player has the commlink factions of all the other human factions, that player may convene the Planetary Council. (When this happens, all the human factions will have each others' comm links.)
The first proposal the Council must consider is electing the Planetary Governor. Only the two factions with the most votes can be eligible. It is advantageous to be the Governor, therefore in some cases it may be best to wait before convening the Council if you haven't enough votes now but have a shot at eligibility within the next few turns.
Proposals.
The primary purpose of the Council is to consider various issues which may arise, such as electing a Planetary Governor.
Elect Planetary Governor.
The Planetary Governor can introduce new proposals more often and enjoys automatic infiltration of all factions. That means if the Governor isn't you, it should be somebody you trust.
Global Trade Pact.
Commerce rates are doubled for all factions. Since choice of faction, social engineering and technologies alter your commerce rating, this proposal can be good or bad accordingly. If your opponents will gain an advantage from increased trading, don't agree to a pact.
Repeal Global Trade Pact.
Commerce rates are halved for all factions.
Launch Solar Shade.
Causes global cooling and the sea levels to drop. If a solar shade has already been launched, this proposal will be called "Increase Solar Shade".
Increase Solar Shade.
"See" Launch Solar Shade
Melt Polar Caps.
Melts the polar ice caps of Chiron, raising sea levels all around.
Repeal U.N. Charter.
Repeals the U.N. Charter, meaning atrocities are counted as normal acts of warfare. The U.N. Charter is in effect when the game starts.
Reinstate U.N. Charter.
Self-explanatory: reinstates the U.N. Charter, once again making atrocities subject to harsh penalties.
Salvage Unity Fusion Core.
Salvage of the Unity core affords every faction 500 energy credits. Logically, you will not want to do this if you have lots of ECs and your rivals are strapped for cash.
Unite Behind Me As Supreme Leader.
All factions unify behind one faction leader, this faction wins and the game is over. When playing with Progenitor in Alien Crossfire, this option is only available after both alien factions are eliminated.