Yes. Your argument that was “monocultural” societies are somehow more peaceful, and now more so than imperial rule, when the history of the 20th century demonstrates the opposite. Nationalism led to massive internal violence and attempts at creating new empires that ultimately led to subjugation of these states between two outside empires.

We're not exactly a post-national society or world yet.

So basically you're arguing in favor of imperialism? Imperialism replaces internal violence with violence (or the threat of it) from an outside oppressor. How is this a good solution?

No, I’m stridently anti-imperialist, but simply observing the historical reality that, counter to OP’s claims, “monocultural” nationalism is a modern, unstable construction, that tends toward violence.

>but simply observing the historical reality that, counter to OP’s claims, “monocultural” nationalism is a modern, unstable construction, that tends toward violence.

You state this as if it's a fact, but it simply isn't. There's lots of monocultural nations that don't have any violence problems whatsoever: Scandinavian nations come to mind immediately.

Moreover, the "golden" days of the Cold War were pretty awful for a lot of people. The US and USSR didn't go to war directly, but instead conducted a bunch of proxy wars: Afghanistan, Vietnam, Korea, etc. The 20th century after WWII was full of violence, it was just kept away from the two imperial powers mostly. Instead they instigated violence in other places to pursue their "interests", such as when the USA overthrew the democratically-elected government in Iran and installed a brutal dictator.

As for stability, I would argue that stability is something we've never had anywhere, not long-term. I don't think you can point to anything that's been continuously stable for a long time, so I would argue that instability is the norm. Trying to keep a society stable is like trying to balance on a beach ball: you can do it for a little while, but eventually you're going to fall.