When you run into other Factions, they will show up in the diplomacy screen. From this screen you can perform some simple but important diplomatic actions, and to respond to diplomatic requests.
Diplomacy in UFO:Afterlight is pretty simple, mostly tied to story rather than ruler attitude or culture ideals. The game expects you to contact Earth, as well as allying with at least one reticulan faction (even temporary), to acquire critical technologies.
It is ok to ally with as much factions as possible, and conquering the whole Mars is equally feasible.
Through diplomatic actions you can gain or give resources with other Mars factions. Once formed, these agreements cannot be broken except by attacking the faction, or until it is wiped out by other factions.
Thus, in the game, be careful not to give out resources, especially Fuel. For the rest you will need at most, , , , and .
You can form alliance with most factions, and can break alliance at any time.
It is impossible to re-establish a broken alliance. But it is possible to refuse alliance request and wage war, and propose alliance at a later time. (Whether they will accept is another matter)
Ally with the Mars factions and they will gift you with a few alien soldiers. Like human soldiers, there is no resupply. When they are gone, they are gone.
The soldiers will need some time to travel to your base. Once they are in, they are your people and under your commands.
When you break alliance with their respective faction, these soldiers will immediately return home.
You can request territories, resources, or items from other Mars factions. Some items or even technologies can only be acquired this way, for example Expedition Battle Armor and Warp Cannon .
You cannot control the exact request. The Mars factions will propose what they are willing to give you and what you must give back in exchange.
You can also request various help from the Earth, but there wouldn't be new characters.
For a complete list of possible deals with Mars factions, please see Diplomacy Deal.
You can also offer help to other factions. Again you cannot control what exactly is offered.