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. While a Casus Belli can be used to start a war, you do not need one to do so - You can declare a Surprise War against another Civilization in Civilization VI who is not your ally.
The more you declare War, the more warmongering points you'll get, which will influence how AI Civs view you.
- Declare Surprise War - Warmonger penalty: Heavy -
- Declare Formal War - Warmonger penalty: Moderate - Used to declare war on a player you have Denounced at least 5 turns ago. No restrictions on which cities you may capture. (Currently it seems to be the "proper" way to go to War with another Civilization before reaching the Renaissance Era.)
Diplomacy is meant to become more complex as the timeline goes on, meaning that in the Ancient Era, you'll be able to declare war easier and with less Warmonger penalties, though later in different Eras they'll start to take effect, and if you're warring with everyone you'll get a large penalty. With the Diplomatic Service Civic in the Renaissance Era you unlock the first four Casus Belli. There are six Casus Belli in total.
- Declare Holy War - Used to declare war on a power that has Religiously converted one of your cities. All Warmonger penalties halved.
- Declare Liberation War - Used to declare war on a power that has captured a city from one of your friends or allies. No Warmonger penalty for liberating any of those cities.
- Declare Reconquest War - Used to declare war on a power that has captured one of your cities. No Warmonger penalties apply.
- Declare Protectorate War - Used to declare war on a power that has attacked one of your allied City-States. No Warmonger penalty for liberating that City-State.
- Declare Colonial War - Used to declare war on a power that is two technological eras behind you.
- Declare War of Territorial Expansion - Used to declare war on a power that borders your empire. Must have 2 of your cities within 10 tiles of 2 of your opponents' cities. Warmonger penalties reduced by 25%. Available with Mobilization civic.