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. Once you reach the Renaissance-era civic Diplomatic Service, you may accumulate Casus Belli against other powers. You must Denounce the other power in order to use a Casus Belli against them. See the table below for the conditions for each Casus Belli, the civic that unlocks them, and the percentage of the usual warmonger penalty that is applied.
Holy War: If one of your cities is converted to the enemy's religion
(Diplomatic Service, 50% warmongering)
War of Liberation: If a friend or ally had cities conquered
(Diplomatic Service, no warmongering penalty when liberating cities that have been conquered)
War of Reconquest: If you had cities conquered in a previous war
(Defensive Tactics, no warmongering penalty when retaking your previous cities)
Protectorate War: if a city-state you are Suzerain of is attacked
(Defensive Tactics, no warmongering penalty when liberating a city-state target)
Colonial War: if target civilization is two eras behind you in technology
(Nationalism, 50% warmongering)
War of Territorial Expansion: if target civilization borders your nation
(Mobilization, 75% warmongering)
Concepts