Travian Kingdoms is a game that is all about cunning diplomacy in a world of warring realms. It's a dog-eat-dog world but there is also plenty of room for cooperation:
Kingdom Unions make it possible for the rulers of two Kingdoms to unite their realms into one entity. This has some practical benefits since both rulers can now share the burden of command and eventually they can more than double the size of their new Kingdom.
But it also comes with a risk: There are now two rulers in one Kingdom who might not always get along when making important decisions. And since all Kingdom Unions last until the end of the world this can lead to some ... lively political struggle.