An Alliance is an agreement between two rulers that they can call upon or join each other's wars via a character interaction. Declaring war on an ally will add -25 Popular Opinion for 3 years.
Creating an Alliance
An Alliance is created automatically between two characters who are married. In addition two Rulers can negotiate an Alliance if one of the following requirements are met:
- Both rulers are at least 4th degree relatives
- Two Close Family members of the two rulers are married to each other
- One of the rulers has the Defensive Negotiations diplomacy lifestyle perk
The acceptance of an Alliance is determined by the difference between the tiers of the two ruler's primary titles, the difference in military power as well as their 
Opinion of each other. In addition the acceptance can be affected by the following:
| Source | Acceptance |
|---|---|
| Terrified | +100 |
| Intimidated | +50 |
| Vassal | +40 |
| Claims | +25 |
| Same House | +15 |
| Heir | +10 |
| Source | Acceptance |
|---|---|
| Rival | -100 |
| Evil Faith | -80 |
| Hostile Faith | -60 |
| At War | -50 |
| Astray Faith | -40 |
| Different Faith | -20 |
A proposed Alliance can be accepted by using a Hook, either Weak or Strong.
Joining Wars
When a character is at war each one of its allies will have the option to join the war via the Offer to Join War character interaction, which the targeted character will always accept. Alternatively a character who is at war will have the option to ask its allies to join a war via the Call to War character interaction. The acceptance is determined by the character's Opinion, modified by the following:
| Source | Acceptance |
|---|---|
| Heir | always |
| Spouse | always |
| Defensive War | +50 |
| Defensive War against their Ally | -25 |
| Offensive War against their Ally | -50 |
| Enemy is Heir | never |
| Enemy is Spouse | never |