Civilization can have alliances with each other. When one of the members of alliances gets involved in a war, they may call their ally to arms.
To create or join an Alliance you need to resort to some requirements:
- Relations: The Relationships you have with the target civilization.
- Government: The most similar are the governments between you and the target civilization.
- Civilization force: The more similar the power among civilizations, the more possibilities of creating the alliance there will be.
- Be in peace: If you are in a war, you CANNOT Join or Create an alliance (Because being at war subtracts 250 points).
- Allies: This can be somewhat confusing, since if the enemies of the objective civilization are your friends, this reiterates points and vice versa.
- Distance Between Borders: This subtracts points as far as civilizations are from each other. It has to be taken as a neighbor for this negative requirement to be removed.
- Want your provinces: This requirement will remove points in case your provinces have core of the target civilization.
- Need Min. 2.0 Diplomacy points
All these requirements (Except for the Diplomacy points) are added or subtracted to give a score, that this will be equivalent in a meter that has to spend at least half in order to make (or join) an alliance.
The alliances remove 0.6 Diplomacy points. Even being in an alliance, you can declare war on one of the members but this would be taken as a betrayal to later leave the alliance automatically.