Before the release of the game, @Wiz made a very valid point about allowing players to join wars: it's tons of fun for the player to intervene in someone else's war, but not very fun to see a war you're winning handily turn into a slog (or worse, a rout) when an AI suddenly jumps on you.So here's my suggestion: keep the joining of the war from being immediate, and give the aggressor empire some control over how quickly the people can jump in.Imagine this: the big-bad empire declares war on its neighbor. Maybe the neighbor has allies, maybe it doesn't, but it's clear that it's going to lose the war. There are two empires concerned about this war: one democratic crusader who's against an autocracy expanding at all, and another empire who just doesn't want the aggressor to take the system that will put their borders in contact.Both empires go to their diplomacy screen with the aggressor and choose an option to Warn them about an existing war. The democratic crusader picks no specific concern, since they want to take any excuse to intervene, while the concerned neighbor picks a territorial concern for that system. A progress bar goes up for both of these warnings.For the democratic crusader who just wants any excuse to get in on the action, they'll receive progress to their bar for stuff like Pops and buildings lost in bombardment, bombardments that go on beyond the complete reduction of defenses, and every time an assault is launched. Basically, if the aggressor goes on too long for doing stuff that is necessary to continue the war, but causes humanitarian issues, they'll have a new defender in the war.The concerned neighbor picked a very specific thing they're worried about though, and in their case their bar will only progress if the enemy operates in or around their chosen area (perhaps you can pick a larger contiguous area than a single system as long as it meets a specific criterion, such as systems populated in part by your species, share a border with you, etc... with some dilution of the effect on the progress bar). Just having ships in the area will add progress, fighting a battle will add more, destroying stations will add still more, bombarding will really get it going, and launching an invasion will pretty much make the bar go to 100%.This way intervention is allowed, but the aggressor knows it's coming and therefore knows exactly where he stands. Rather than conquering the entire empire, he might just take a couple of systems (staying well clear of the concerned neighbor's area of interest), giving him a clean win that just wasn't as big as he expected it to be. Say the progress bar will allow him to safely take two of his target's five systems: he can see this and take them, rather than getting up to four systems and being dogpiled by two empires each as big as he is (which would be).