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Casus Belli is a new gameplay mechanic introduced in Civilization VI that allows you to justify your wars and get less warmonger penalties if you meet certain prerequisites. 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.
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 form of war for the early game. During any post-Ancient Era, you will receive full warmonger penalties, and there are no requirements.
Formal War Edit
This is the first Casus Belli you can use to justify a war. For this, 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 would 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 Civic. 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 would be the type of war you are going to be using most often.
Liberation War Edit
This Casus Belli is unlocked with Diplomatic Service Civic. It is used to declare war on a power that has captured a city from one of your friends or allies. 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 Civic. It is used to declare war on a power that has captured one of your 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 Civic. It is used to declare war on a power that has attacked one of your allied city-states. No warmonger penalties 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 Civic. It is used to declare war on a power that borders your empire. Must have 2 cities within 10 tiles of 2 opponents' cities. Warmonger penalties reduced by 25%. As the title explains, this would be used to expand your empire when things start to get crowded, and can also be incredibly useful in domination games.
Colonial War Edit
This Casus Belli is unlocked with Nationalism Civic. It is used to declare war on a power that is two technology eras behind you. All warmonger penalties 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.