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 less 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 full warmonger penalties for your current era.
Formal War Edit
This is the first Casus Belli you can use to justify a war. For this, you must either have been already denounced by the other leader, or you must have denounced the leader at least 5 turns prior to declaring war, essentially giving them time to prepare. There are no restrictions on which cities you may capture. If it is the Classical Era or Medieval Era, this is probably going to be the type of war you will want to use in order to avoid heavy warmonger penalties.
This Casus Belli can also be gained if a nation breaks a promise given to you.
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. All warmonger penalties are halved. 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. You gain no warmonger penalties for liberating any of these cities. 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 used to declare war on a power that has captured one of your cities. No warmonger penalties apply, as long as you do not attack any other cities. 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. No warmonger penalties apply for liberating the city-state. Again, you can use this if another force is becoming too powerful and is beginning to take down your neighbors.
War of Territorial Expansion Edit
This Casus Belli is unlocked with Mobilization. It is used to declare war on a power that borders your empire. You must have 2 cities within 10 tiles of 2 of the opponent's cities. Warmonger penalties are reduced by 25%. As the title explains, this will be used to expand your empire when things start to get crowded, and can also be incredibly useful in domination games. The problem is that the warmonger penalties are not reduced much.
Colonial War Edit
This Casus Belli is unlocked with Nationalism. It is used to declare war on a power that is two technology eras behind you. All warmonger penalties are halved. 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. Again, Domination Victory seekers might find this useful when mopping up the weaker nations around the globe.