^I'm leaving that up there for posterity and writing it down naming names since otherwise it's confusing and sounds weird. The protagonist (King Arthur) has a neighbouring Kingdom called Strathclyde, originally founded by a man named Ceretic. one royal branch, living in the left portion, are the descendants of his son Cedic. In the right portion are his decendants through his son Erbin. Traditionally a folk-moot decided their king. It could be any pretigious warrior but it alternated every five years between a member of the opposing branches. While it sort of worked, eventually Caw (of the Erbin branch) realised his likely successor Tudwal was a complete moron unfit to rule. So he decided to nix their "democracy" and declare himself absolute ruler, dissolving the moot, he immediately abdicated in favour of his son (he has 24 sons). The left portion have revolted against him, and gotten some "foreign allies". One is a cousin of Tudwal and descendant of Cedic. The other two are Kings closely related to the Cedic branch through marriage and basically want to please their wives. But Arthur has no reason to fight, and I'm stumped on why he even would.
Maybe all the kingdoms have a shared enemy or problem that can only be defeated if they work together? Or maybe Arthur is using some Machiavellian strategy to usurp power by pretending to be an ally?