同盟
同盟（Alliance）とは、2人の支配者（rulers）がキャラクターの関係行動（interaction）によって互いの戦争を呼びかけたり参加したりすることができる協定であり、一方が封臣（Vassal）である場合は、犯罪（crime）を犯していない限り称号を剥奪することはできません。同盟国に宣戦布告すると3年間は領民の評価（Popular Opinion）に-25が追加されます。同盟を作成するための要件が満たされなくなった場合、進行中の戦争は影響を受けませんが、同盟は自動的に終了します。
同盟の作成
結婚した2人のキャラクターの間には自動的に同盟が作られます。また2人の支配者の近親者（close family）同士が結婚または婚約した場合にも自動的に作成されます。婚姻締結画面で婚姻が同盟につながるかどうかを確認することができます。また支配者は同盟交渉をすることもできます。
同盟交渉
まだ同盟しておらず、投獄されていない統治者は、次の2つの関係行動を介して同盟を交渉することができます。
- 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.