Need help/advice: making countries set long after WW3 devastated the world

Hi, I’m toying with this “high tech, low life” concept for an Earth that has slowly rebuilt after a global war, but basically none of the Old World countries exist anymore.

Either they were lost to history, or they were deliberately forgotten by the survivors.

Instead, whole new countries were established, with only vague artifacts of the nations they were descended from.

These new countries are currently locked in a new world war, fought with the advanced technology leftover or built off of the previous war.

So, I guess my question is:

1) Is that realistic?

2) Is there a way to Worldbuild new nations (without stereotyping)?