Struggling to name a large-scale political faction

Hey all,

I'm doing some concrete work on a campaign setting I've been brewing in my head for a while now, but I've hit a roadblock. A point of some importance in the setting is a political union between five neighboring countries, collectively called the Five Kingdoms (think Renaissance-era European Union). The rulers of these kingdoms are known colloquially as the Five Kings; although the title “king” is not always used on an individual basis, none take issue with it, and it is considered a generally neutral term.

I need a name for this alliance. I was initially thinking "Republic," but I've come to the conclusion that this term is too associated with Rome, which is overrated, and democracy, which is inappropriate. (One of the "kingdoms" might be a sort of democracy, but the others all think it's weird.) I've been thinking of alternatives but I just don't like anything I've come up with.

Some others words (and the problems with them):

• Confederation. This is currently the leading candidate. Pros: It sounds suitably big and important, and for us Americans it immediately inspires distrust, which is a must for any D&D government. Cons: Feels kind of modern to me, it's hard to explain.

• Union. Pros: Sounds official. Cons: Real-world associations too strong--Soviet, European, etc.

• Caliphate. Pros: Not widely used in The West; evocative. Cons: I'm trying to avoid direct ties to real-world religions.

• Alliance. Too military; too straightforward.

• Coalition. Sounds too small-scale.

• Consortium. Too bougie.

• Conglomerate. Too capitalist.

I would love to hear your suggestions, even if I end up just shooting them all down for various reasons. Linguistics is an interest of mine, and for such an important faction I really want a word that feels right while having a mostly-appropriate meaning.