Casus Belli is a new gameplay mechanic introduced in Civilization VI that allows you, after you meet certain prerequisites, to justify the wars you declare and get fewer warmonger penalties. The alternative to using a Casus Belli is the Surprise War, which will give you a much larger warmonger penalty than the Casus Belli, except in the Ancient Era in which there is no penalty. As the game goes on and you enter new eras, the warmonger penalties increase on both Surprise War and Casus Belli.
Most Casus Belli are unlocked by researching specific civics, as detailed below. As of the Spring 2018 Update, Casus Belli can be used when declaring a Joint War with another civilization. However, there are diplomatic situations in which Casus Belli cannot be used, such as when a leader protests your positioning of troops on his/her borders, and you decide to answer by declaring war.
Types of Casus Belli Edit
Surprise War Edit
While technically not a Casus Belli (as they are defined as justifying a war and Surprise Wars are not justified), this is the standard (and only) form of war for the early game. There are no requirements for declaring a Surprise War - you can do it immediately, whenever you please. Post-Ancient Era, however, you will receive 50% greater warmonger penalties for such an outrage, which makes this the worst possible choice from a diplomatic standpoint.
Formal War Edit
This is the first Casus Belli you can use to justify a war. A Formal War can only be declared after some diplomatic incidents (such as denouncing or being denounced by another leader at least 5 turns prior, or having a leader break a promise that he or she made to you), and as such comes as no surprise to any party. There are no restrictions on which cities you may capture. If it is the Classical Era or Medieval Era, this is the type of war you will want to use in order to avoid heavy warmonger penalties. The warmonger penalties for this type of war are standard.
Holy War Edit
This Casus Belli is unlocked with Diplomatic Service. It is used to declare war on a power that has religiously converted one of your cities, thus sparking religious outrage among your citizens. Given the importance of religion in this game, the other leaders tend to become more understanding of such wars. All warmonger penalties are halved (-50%). If you are playing for a Religious Victory and have competition trying to convert your land, this will be the type of war you are going to be using most often.
Liberation War Edit
This Casus Belli is unlocked with Diplomatic Service. It is used to declare war on a power that has captured a city from one of your friends or allies. Declaring war in such case is seen as justified in any Era, and you will gain no warmonger penalties as long as you liberate any of these cities, and for the declaration of war itself. This acts as a retaliatory war of some sort; if you have a strong ally that has been weakened, then you can use this to help them get back on their feet.
Reconquest War Edit
This Casus Belli is unlocked with Diplomatic Service. It is very similar to a Liberation War, but this time it relates to your own cities - as long as you declare war on the power that has captured one (or more) of your cities, and you do not capture any other cities, no warmonger penalties apply. This can also be used as a form of retaliation, or a way of justifying the reentering of a war that ended previously.
Protectorate War Edit
This Casus Belli is unlocked with Diplomatic Service. It is used to declare war on a power that has attacked one of your allied city-states. As is the case with Liberation and Reconquest Wars, other leaders understand and admire this kind of loyalty, and no warmonger penalties apply for either declaring the war or liberating the city-state. Again, you can use this if another force is becoming too powerful and is beginning to take down your neighbors.
Colonial War Edit
This is one of the more modern Casus Belli, unlocked with Nationalism. It is used to declare war on a power that is two technology eras behind you. Other leaders accept such a war as a way to "bring civilization" to an otherwise barbaric and backwards country. All warmonger penalties are halved (-50%). This can be used effectively against empires that have been focusing on a different element, such as their military, and gives power to more technologically advanced civilizations. Domination Victory seekers might find this useful when mopping up the weaker nations around the globe.
War of Territorial Expansion Edit
This is the last Casus Belli available, unlocked with Mobilization. It is used to declare war on a power that borders your empire, and in whose lands you wish to expand. You must have 2 cities within 10 tiles of 2 of the opponent's cities for this Casus Belli to apply. Warmonger penalties are reduced by 25%, since leaders frown upon expansionism, but can understand it somewhat. As the name suggests, this will be used to expand your empire when things start to get crowded, and can also be incredibly useful in domination games.
Rise and Fall Expansion Edit
With the Rise and Fall expansion, some new Casus Belli have been introduced.
Golden Age War Edit
This Casus Belli is available if the player opts for the 'To Arms!' dedication upon entering a Golden Age. There are no restrictions and all warmongering penalties are reduced by 75%, which arguably makes it the best Casus Belli for a Domination Victory.
However, the specific dedication is available only after the Industrial Era and requires the player to be in a Golden Age.
War of Retribution Edit
This is one of the first Casus Belli in the game, unlocked with Early Empire. It is used to declare war on a power that has broken a promise that was made to you within the last 30 turns. During a War of Retribution you have a 50% reduction of standard warmonger penalties for the current era for declaring war and capturing cities, but razing cities gives you a 200% 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 Edit
This Casus Belli is unlocked with Ideology. It is used to declare war on a power with a different Tier 3 government. Doing so grants you a 50% reduction in the warmonger penalties for declaring war, capturing cities and razing cities. The Ideological war is one of the most suitable for late game conquests, since its only prerequisite is for the target to have a different government, and this is very often the case.
A Case for War
Declare a formal war