It's not so much about normal as about the point of the metaphor. I don't think they generally shouldn't be used; I think this one doesn't really work in this context because international relations aren't nearly as "exclusive" as relationships, not can they ever be completely separated.

If we want to make an analogy about social relationships applied to international relationships, you're still moving in the same social circles where a friendship or enmity with someone ripples out into the dynamics of your relationship to everyone else. They might approve of your choice of friends or look critically at your closeness to someone they can't stand, feel like they have to choose a side or appease both of you, defend or condemn either party's actions...

It's not like you'd form new relationships upon one breaking down if you already know everyone else and everyone also knows both you and your ex.