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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[edit]

An Alliance is created automatically between two characters who are married, it is 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. In addition, two Rulers can negotiate an Alliance if one of the following requirements are met:

  • The ruler is your child, parent, or sibling
  • You are married to a child, parent, sibling, grandchild, grandparent, uncle, aunt, niece, or nephew of theirs
  • They are married to a child, parent, sibling, grandchild, grandparent, uncle, aunt, niece, or nephew of yours
  • Their child, parent, or sibling is married to a child, parent, or sibling of yours
  • 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[edit]

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 ( Prestige)
Baron 750[1]
Count 75
Duke 150
King 350
Emperor 750

Allies declining a call to war invitation will suffer an opinion penalty with their ally (-10 for one year for offensive wars; -20 for three years for defensive wars).

End of the Alliance[edit]

The alliance ends immediately after the requirements listed in "Creating an Alliance" are not fulfilled anymore. Ongoing wars are not affected.


  1. call_ally.0100 in game\events\interaction_events\alliance_interaction_events.txt . Barons, along with empires, cost massive_prestige_value to call (likely a bug).
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