Casus Belli represent reasons to go to war where the normal Warmonger penalty for a declaration of war or capturing cities are reduced or eliminated. Once you reach the Renaissance-era civic Diplomatic Service, you may accumulate Casus Belli against other powers. You must Denounce the other power in order to use a Casus Belli against them. See the table below for the conditions for each Casus Belli, the civic that unlocks them, and the percentage of the usual warmonger penalty that is applied.

Holy War: If one of your cities is converted to the enemy's religion
(Diplomatic Service, 50% warmongering)

War of Liberation: If a friend or ally had cities conquered
(Diplomatic Service, no warmongering penalty when liberating cities that have been conquered)

War of Reconquest: If you had cities conquered in a previous war
(Defensive Tactics, no warmongering penalty when retaking your previous cities)

Protectorate War: if a city-state you are Suzerain of is attacked
(Defensive Tactics, no warmongering penalty when liberating a city-state target)

Colonial War: if target civilization is two eras behind you in technology
(Nationalism, 50% warmongering)

War of Territorial Expansion: if target civilization borders your nation
(Mobilization, 75% warmongering)

Casus Belli represent reasons to go to war where the normal warmonger penalty for a declaration of war or capturing cities are reduced or eliminated. The first Casus Belli becomes available after you research Early Empire, and others become available as you research other civics. You must Denounce the other power in order to use a Casus Belli against them. Each of these are laid out below, along with the details of their warmonger penalties.

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 diplomatic penalty for declaring the war or capturing enemy cities (keep in mind this diplomatic penalty increases during the game.) Razing an enemy city carries three times the penalty from simply capturing the city.

Surprise War: You can declare a Surprise War at almost any time, but doing so carries severe warmongering penalties, typically an additional 50% of the warmongering penalty of a Formal War in the current era. Capturing cities likewise have an additional 50% penalty, and razing an enemy city during a Surprise War is particularly heinous—it carries a warmonger penalty of 450% for the era.

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 warmonger penalty applies to the declaration of war, capturing cities, or razing cities. If you wish to raze enemy cities, a 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 warmongering penalties as a Formal War in the 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 carries no warmonger penalty. You may also choose to raze your reconquest target 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 warmonger penalty for declaring war on your target, but you will suffer the era’s warmonger penalty equal to a Formal War if you capture an enemy city. Razing an enemy city during a Protectorate War carries three times the era’s warmonger penalty. You cannot use the aggression of another civ against a third party as justification for a 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 warmonger penalty for declaring war on your target. You will suffer the era’s warmonger penalty equal to a Formal War if you capture another, different enemy city, and if you raze an enemy city, you will suffer 600% of the era’s warmonger penalty! This is the most diplomatically costly warmonger penalty, as burning cities in the name of freedom is particularly awful to the other civilizations of 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 only 50% of the warmonger penalty for the era in declaring war and capturing cities. If you raze a city, you will receive the full warmonger penalty as a Formal War for the era, as the world no longer buys your excuses about “bringing enlightenment” to your target.

Territorial War: You may declare a 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 warmonger penalty by 25% for declaring war and capturing cities. Razing cities carries a penalty of 50% of the warmonger penalties for doing so under a Formal War. This Casus Belli is useful if you wish to eliminate enemy cities that are located 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 warmonger penalties and city capture penalties are only 25% of the warmonger penalty for the era, making this one of the least diplomatically costly ways to grab territory from your rivals. The warmonger penalty for razing a city is the same as in a Formal War—300% of the normal warmonger penalty for the era. 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 you have a 50% reduction of the Formal War warmonger penalties for the current era for declaring war and capturing cities, but razing cities will give you a 200% penalty for the current era’s warmonger penalty. 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 you have a 50% reduction in the warmonger penalties of the era for declaring war, capturing cities, and razing cities.