How I picture this working in a civ 6 style engine: To unlock colonial wars (not the same as the current colonial war CB... let's call the current one "colonization" war), you need to be in a renaissance government. You declare colonial wars with a stipulation attached that limits it to one continent, which cannot be the home continent of you OR your target. Fighting is only allowed on that continent (including its oceans). Example using real world maps: England declares a colonial war on the Netherlands with the continent America. Boston (an English city) can now try to conquer New York/New Amsterdam (a Dutch city) and vice versa, but English and Dutch troops can't fight in Europe (unless a regular war is declared). You get only small warmonger penalties with civs on your home continent, but very large ones with any civ on the continent where the target is. Proxy wars are similar, except instead of choosing a continent, you choose someone with a declared allegiance. Example with real world geography, and a somewhat nonsense situation: Spain is allied with the Aztecs. England is allied with the Cree. England makes an in-game deal with the Cree to declare a proxy war on the Aztecs. Cree troops can now attack Aztec troops and vice versa. England and Spain cannot attack each other directly (unless with a regular declaration of war). This opens a city project in both civs to "send forces", like the send aid project. When the project is completed, their allied civ spawns a unit in each city, or something like that. Warmonger penalties go with ideologies in this case.