Since the Peace of Westphalia in 1648, the world has been increasingly divided into and defined by nation-states.
It was then that the idea of countries with fixed borders, recognised rulers, and defined citizenship took root.
Before that, you might live near a village not far from a city and a Lord might come through from time to time and demand allegiance and tribute. His loyalty to one crown or another was a separate question.
After that, you might live 5 miles from the border of the Kingdom of Ruritania, and that made you a Ruritanian until the end.
Since then, we've had that system of defined borders, rulers, and citizens. There are some variations on it: the multinational state (Canada, UK, Swizerland), the stateless nation (the Kurds), the federal state (Australia, USA, Canada) but it is all variations on the same structure.
The thing is, it has only been with us for a few centuries. Who can say if it will last?
Do you think the future holds more of the same? Will countries break apart into city-states? Will the UN be replaced by a global government? Regional empires? Or something previously unimagined?