Over the course of centuries and millennia, some nations die and other nations are born. That's how it's always worked. But for some reason, something's changed. Even though nations are still dying, no new nations are born to take their place. At first, it seems natural - after all, nations have died before, and new nations always show up eventually. But as no new nations show up, the nations gradually realize that they are going extinct, and that the world is evolving into a place where nations exist only as citizens on territory with governments, not as personifications - just like the world we have in real life.

I'd prefer it with modern nations gradually dying out, but you can do it whatever way you'd like.