Crusader Kings III – Casus Belli Guide

A “casus belli” (CB), meaning “Cause for War” in Latin, represents a recognized and legitimate justification for initiating war in a game. It is essential for a ruler to have a valid casus belli to declare war, and the potential gains from the war are explicitly defined by the casus belli.

Using a casus belli comes at a cost of Prestige and/or Piety. If a character is unable to pay the cost, they can instead use a Level of Prestige or a Level of Devotion. Notably, declaring war against a religious leader costs 500 Piety if the leader is spiritual, and 100 Piety if temporal. Some game rules can eliminate the costs associated with a casus belli.

Independence War: The target is the liege. The cost is 500 Prestige for non-tribal and 250 for tribal rulers. Successful war results in the attacker’s independence. A white peace or surrender leaves the attacker as a vassal, but there could be additional penalties such as imprisonment or title revocation.

Seize the County: The target must be within a de jure duchy. The cost is 100 Prestige. A victory results in the attacker gaining the targeted county. In the event of a white peace or surrender, the defender retains their titles, though the attacker may need to pay war reparations.

Seize the Duchy: The target is within a de jure kingdom, costing 100 Prestige per county in the target duchy. A successful war grants the attacker the targeted duchy. If there’s a white peace or surrender, titles remain with the defender, and the attacker might pay reparations.

Claims: Costs vary based on the claim’s level and whether it belongs to the claimant or another party. They range from 100 Prestige for a personal county claim to 3000 for an empire-level claim on behalf of another. Winning the war grants the claimant the titles, while a white peace or surrender leaves the titles with the defender and the claimant may lose the used claims.

Subjugate: Applicable under specific governance or religious conditions. The cost is 1000 Prestige for non-tribal and 500 for tribal rulers. Victory makes the attacker the liege of the defender. In the event of a white peace or surrender, the defender remains independent, but the attacker may face discontent within their factions.

Raid for Captives: The faith must have the Human Sacrifice tenet, with a cost of 80 Prestige if the faith has the Armed Pilgrimages tenet and 100 otherwise. Successful war results in the attacker imprisoning 3-5 of the defender’s courtiers. No prisoners are gained in a white peace or surrender scenario.

Excommunication War: The target must be of the same faith and have the Excommunicated trait. There is no cost. Victory results in the defender being deposed and losing the Excommunicated trait, while the attacker loses Opinion with the Head of Faith. In the event of a white peace or surrender, titles remain with the defender.

Ducal Conquest of County: Requires the Conquest lifestyle perk. The cost is 350 Prestige for targets with a county title rank and 750 for higher ranks. A victorious attacker gains the targeted county. Titles remain with the defender in the event of a white peace or surrender, but the attacker may face internal discontent…

Forced Vassalization: Requires the Forced Vassalage lifestyle perk, costing 125 Prestige per county in the target. Victory makes the attacker the liege of the defender. A white peace or surrender leaves the defender independent, but the attacker may face discontent from their factions.

Laudably Conquer County: Requires the Request Claim on Ireland decision. Costs vary: 150 Prestige and 100 Piety for feudal rulers of a different faith, 75 Prestige and 50 Piety for tribal rulers of a different faith, or half those amounts for rulers of the same faith. Victory results in the attacker gaining the defender’s titles, while titles remain with the defender in the event of a white peace or surrender, and the attacker may face internal discontent.

Mongol Invasion of Kingdom: Requires the Mongol Empire title and a tribal government, with a cost of 200 Prestige. Victory grants the attacker the targeted kingdom. In the event of a white peace or surrender, titles remain with the defender, and the attacker may face discontent from their factions.

Restore Imperial Province: Requires the Roman Empire title, a target duchy that was part of the Roman Empire and a Christian or Greco-Roman religion. There is no cost. Victory results in the attacker gaining the targeted duchy. Titles remain with the defender in the event of a white peace or surrender, but the attacker might need to pay war reparations.

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