同盟
An Alliance is an agreement between two rulers that they can call upon or join each other's wars via a character interaction and cannot revoke titles if one is a vassal unless guilty of a crime. Declaring war on an ally will add -25 Popular Opinion for 3 years. If the requirements to create an alliance are no longer met the Alliance will be ended automatically although ongoing wars are not affected.
同盟の作成
Alliances are created automatically between two characters who are married. They are also created automatically when two close family members of the two rulers are married or betrothed to each other. It is possible to see whether the marriage proposal will result in an alliance on the marriage proposal screen. Rulers may also negotiate alliances.
同盟の交渉
Rulers who are not already allied and are not imprisoned may negotiate an alliance via two interactions:
- Negotiate Alliance, if the rulers are family members through blood or marriage.
- Propose Alliance, unlocked by the Defensive Negotiations perk under the
Diplomat lifestyle tree.
Note that if two rulers may use the Negotiate Alliance interaction, the Propose Alliance interaction is not shown.[1] Furthermore, if the recipient has rejected an offer of alliance from the proposer in the past 100 years, an alliance may not be offered.[2]
Negotiate Alliance interaction
The Negotiate Alliance interaction may be used to create alliances if the following conditions are met:
- Both rulers are not at war with each other.
- The recipient is not an abductor or abductee.[3]
The proposer is count rank or above.[1]- The rulers are family members, if one is the other's:
- Parent/child
- Sibling
- Spouse
- Spouse of close and extended family, excluding those related only as cousins.[4]
- Alternatively, they may be related through marriage ties if one's child, parent or sibling is married to a child, parent or sibling of the other.[5]
Propose Alliance interaction
Characters may use the Propose Alliance interaction if they have the Defensive Negotiations perk and are unable to use the Negotiate Alliance interaction. Vassals and lieges may not propose alliances with each other. Additionally, they must meet the following requirements:
- Both rulers are not at war (not necessarily with each other).
- The recipient is not an abductor or abductee.[3]
- The recipient is count rank or above.[1]
Alliance acceptance
The acceptance of an alliance is determined by the target's 
opinion, the difference in rank, the difference in military power and your dynasty's level of splendor. The following traits affect acceptance as well:
| Source | Acceptance |
|---|---|
 Terrified
|
+100 |
 Intimidated
|
+50 |
 Vassal
|
+40 |
| I am a claimant | +25 |
| Same House | +15 |
 Heir
|
+10 |
| Source | Acceptance |
|---|---|
| Is a claimant | -200 |
| 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 their 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 their 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 |
There is no prestige cost to call on an ally to assist in a defensive war. However, there is a cost for offensive wars that is based on the ally's rank:
| Rank |  Cost
|
|---|---|
 Baron |
750[6] |
| Count |
75 |
 Duke |
150 |
 King |
350 |
 Emperor |
750 |
Allies declining to join an offensive war lose -20 Opinion for 5 years and 
350 Fame. Allies declining to join a defensive war lose -50 Opinion for 25 years and 750 Fame.
If a war lasts at least 2 years and one of the allies has 0% War Contribution, it will lose -20 Opinion with the war leader and have to pay Gold or Prestige or promise to earn War Contribution within a year. Breaking the promise will result in losing a Level of Fame as well as -50 Opinion with the war leader.
References
- ↑ 1.0 1.1 1.2
game\common\character_interactions\00_alliance.txt - ↑
game\common\opinion_modifiers\00_marriage_opinions.txt,refused_alliance_opinion - ↑ 3.0 3.1
game\common\scripted_triggers\00_available_for_events_triggers.txt,is_diplomatically_available - ↑ Therefore, spouse of one's parent, child, sibling, grandchild, grandparent, uncle, aunt, niece or nephew.
- ↑
game\common\scripted_triggers\00_family_triggers.txt,is_any_family_relation_or_spouse_trigger - ↑
call_ally.0100ingame\events\interaction_events\alliance_interaction_events.txt. Barons, along with empires, costmassive_prestige_valueto call.