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. Note that Casus Belli cannot be used in certain diplomatic situations, such as when declaring a Joint War with another civilization, or 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. During any 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.