Only two ways the peace fires: If the war ends in a peace deal without the "Religious Supremacy" demand enforced (which AFAIK, the AI will *never* do, the main attacker/defender will never offer a peace deal to their opposite that doesn't have that demand, meaning you have to either be the HRE or, more likely, in charge of the Protestant League for it to ever happen) or if it manages to last long enough for the event to fire, (which I think takes several decades in-game) Both are practically luck-based, no idea how to make sure you are the leader of the Protestant League (who is the only one that can start the League War) and succeeding in the dance required to make the war last long enough for the peace on either side is a nightmare.
I can understand *why* that's the case -doing the united HRE achievement pretty much requires the ability to enforce religious unity, and the nature of the leagues means you can't rely on a player always being in a position to say whether to go all the way or not- but I really think you should have to work hard to make a Protestant or Catholic HRE a thing, not game the system to force an HRE where any Christian can be elector or rule.
