- study the "Diplomacy" science,
- build the Diplomatic Corps building,
- offer an alliance to other player and wait for the player to accept the offer, or yourself accept offer from another player.
Alliance considered formed right after the offer acceptation.
Alliance rupture takes 24 hours. Alliance rupture cannot be cancelled.
You can allow your ally to see your domains or do not, as well as missions.
A player can have up to 5 allies.
Maximal number of allies at the moment depends on the "Diplomacy" science level.