The modern idea of a state, with involatile borders and exclusive sovereignty on its land, people and agents abroad, is popularly traced back to the Peace of Westphalia [1]. At least as much as such an idea can be traced to any singular event.
It also caused the end of theocracy in Europe. And of course the massive population declines (over 50% in some regions) lead to all sorts of social changes. Without all of this, the Age of Enlightenment, which was just starting in the final years of the Thirty Year's War, might have played out differently across Europe.
1: https://en.wikipedia.org/wiki/Westphalian_system