Casus Belli is the Latin expression meaning "a case for war." In law, these are legally recognized reasons for nations to go to war with one another. In Civilization VI, they are your stated reasons for going to war with another civilization, and they affect other civilization's attitude towards you, as reflected by Grievances in diplomacy (see the Grievances section as well.)

Casus Belli represent reasons to go to war where the Grievance penalties for actions are reduced or eliminated. The first Casus Belli becomes available after you research Early Empire, and others become available as you research other civics. They are laid out below.

Formal War: You may declare a Formal War after Denouncing an enemy and waiting for 5 turns, after which the Formal War option will be available to you in the diplomacy screen with your target. Formal War carries the current era's standard Grievance penalties.

Surprise War: You can declare a Surprise War at almost any time, but doing so carries severe Grievance penalties, typically an additional 50% of the Grievance generated by actions during a Formal War. Razing cities during a Surprise War carries an especially grievous Grievance penalty.

Holy War: You may declare a Holy War if your religion differs from that of your target, you've researched the Diplomatic Service Civic, and you've denounced your target and waited 5 turns. During a Holy War, only 50% of the current era's Grievance penalty applies. If you wish to raze enemy cities, Holy War is one of the least diplomatically costly ways to do so.

Joint War: You may declare a Joint War with another civilization against a target. Both partners in the Joint War need to have researched the Foreign Trade Civic. The Joint War agreement lasts for 30 turns, and carries the same Grievance penalties as a Formal War in that era.

Reconquest War: You may declare a Reconquest War if another civilization has captured a city that you founded, and have researched the Defensive Tactics Civic. You must Denounce your target and wait 5 turns before declaring war. This Casus Belli is unique in that it generates no additional Grievances against you. You may also choose to raze your reconquest city without penalty if you successfully capture it. Perhaps you cannot bear the sight of your retaken city? You're the implacable tyrant; do what you want.

Protectorate War: You may declare a Protectorate War if one of your allied city-states is targeted for war by a major civilization, and if you have researched the Defensive Tactics Civic. You must denounce your target and wait 5 turns before declaring war. There is no Grievance penalty for declaring war on your target, but you will suffer the era's Grievance penalty equal to a Formal War if you capture an enemy city. Razing an enemy city during a Protectorate War carries an additional Grievance penalty, as the world does not recognize your Protectorate War as legitimating your land grab.

Liberation War: You may declare a Liberation War if a friend or ally had cities conquered, and if you have researched the Diplomatic Service Civic. You must denounce your target and wait 5 turns before declaring war. There is no Grievance penalty for declaring war on your target. You will suffer the era's Grievance penalty equal to a Formal War if you capture another, different enemy city. If you raze an enemy city, you will suffer a very severe Grievance penalty! This is the most Grievance-costly action in the game, as burning cities in the name of freedom is particularly awful to the other civilizations in the world.

Colonial War: You may declare a Colonial War against a civilization which is lagging behind you by 2 technologies, and you have researched the Nationalism Civic. You must denounce your target and wait 5 turns before declaring war. This Casus Belli has a reduced Grievance penalty for the era, both for declaring war and capturing cities. If you raze an enemy city, you will receive the full Grievance penalty for a Formal War of the era, as the world no longer buys your excuses about "bringing enlightenment" to your target.

Territorial War: You may declare Territorial War against a civilization if you have researched the Mobilization Civic. The target must have 2 cities within 10 tiles of 2 of your cities. You must denounce your target and wait 5 turns. This Casus Belli reduces the era's Formal War Grievance penalty by a bit, both for the actions of declaring war and capturing cities. Razing cities carries a reduced penalty under this Casus Belli, making it useful if you wish to eliminate enemy cities in regions where you wish to expand.

Golden Age War: You may declare a Golden Age War if you have chosen the To Arms! Dedication, and denounced your target. You do not need to wait after denouncing your target to have access to this Casus Belli. During this time your Grievance penalties for the war declaration and city penalties are greatly reduced, making this one of the least diplomatically costly ways to grab territory from your rivals. However, razing an enemy city will have the same Grievance cost as in a Formal War. Having a Golden Age entitles you to the respect of other nations, but not license to act with impunity.

War of Retribution: You may declare a War of Retribution if you have researched the Early Empire Civic, and another civilization has broken a promise they made to you within the last 30 turns. You must denounce your target and wait 5 turns before declaring war. During a War of Retribution only 50% of the current era's Grievance penalty applies. Although an enemy may have broken his promise to you, this does not entitle you to burn his cities to the ground in revenge.

Ideological War: You may declare an Ideological War if you research the Ideology Civic, and have a different Tier 3 government from your target. You must denounce your target and wait 5 turns. During an Ideological War only 50% of the current era's Grievance penalty applies.