The modern theory of sovereignty had been developing since the mid-16th century, but it’s probably fair to say that the Peace of Westphalia was the first significant set of treaties to use that theory as its framework. Practically speaking, though, states like England and France had been operating on a lot of these assumptions for some time, so I don’t think it’s correct to say that the treaties “established” the nation-state idea. The major change, as I said earlier, is that states are for the most part agreeing not to intervene in each others’ religious affairs.
