Coalition (war)

+1

I'm not sure that's the way I'd like to see it in game, but I agree that we need joint wars further down the line.

+1. Still, I think coalitions should be unlocked in the tech tree after researching "centralized power". Personally speaking, I would prefer that coalitions replace the "humankind congress" in the early modern period. The winning side should dominate in deciding civic matters, just like the Westphalia system and Congress of Vienna (by the way, although it was called a congress, it was not a UN-like organization but a peace conference after the anti-Napoleon coalition). The humankind congress, which replicates the League of Nations and UN, should be delayed until at least the end of the industrial age, or after researching trench warfare. On the other hand, we could also borrow some rules from EU4. For example, if an empire expands too quickly, upsetting the balance of power, the remaining players (no matter how they feel about each other, though if coalition members dislike one another greatly, they would not fight an honest battle with you, instead leaving the heavy work to the leader and focusing on gaining war score and occupying cities) should feel threatened by that empire and form a coalition to confront it. They would demand either the return of cities, territories or war reparations.

I agree with the technological requirement.

I believe that the "perceived threat" modifier to diplomacy already exists and can easily be fine-tuned to suit this addition.

Pre-Congress coalitions would form around alliances and may cause some to collapse (refusing a call to arms should break an alliance), but an enduring alliance block would effectively act as a de facto "Congress of Vienna" equivalent. I'd welcome additional features interacting with coalitions mechanics, like a building (infrastructure or district or wonder) granting bonuses when part of a coalition (in addition to bonuses for having allies, now that allies are harder to keep and being allied is not a requirement to join a coalition).

I'll also add that Victoria2 (which had a coalition and crisis system that can lead to world wars or peaceful resolution if a great power intervenes) had a "cut down to size" casus belli that was meant to allow nations to declare war on leading powers for no other reasons than their size makes them a threat to world peace or something. Winning a war with that as your demand could force the surrendering party to disband military units until a reasonable threshold is met. I don't think that over-expansion should allow others to demand territories they have no influence (cultural or religious) over, but anyone bordering such territories probably already have claims on those and thus have incentives to join a coalition. I think that introducing such coalitions in the game would warrant such new grievances/demands to complement them.