The system of nation-states we have today is occasionally called the Westphalian system. My understanding is that this revolves around mutual recognition of statehood, which seems like a reasonably elegant approach to the definition of something as otherwise hard to define as a state. However, it seems that some states - England and France, for example - had national identities from long before this. How much of the Westphalian treaty was new thinking, and how much was simply writing down what was already done? Or, indeed, is attributing this to the treaty of Westphalia accurate at all?
A reply to /u/gothwalk
This is a very good question. The term "Westphalia" has been used to describe the modern nation-state system, although it did not fully define the system as we know it today. In reality, Westphalia was a significant diplomatic effort where the process of defining diplomacy was as important as its outcomes. Thus, it serves as an important milestone and the name stuck.
In Westphalia, that nations such as France and Spain were sovereign was not new. What was new was that small nations were sovereign too, and had their own voice. This was very important in the context of the member states of the HRE.
Further, as you know the idea of religious choice and religious establishment was changed and re-defined. No longer was the Roman Catholic Church ever present in all of Latin Christianity, each ruler could choose his/her own church. This did not quite lead directly into separation of church and state, nor universal religious freedom; but it led to independence of the churches or each state as they saw fit.
A corollary of the two was the concept of integrity and non-intervention. The 30YW broke out due to intervention of the HRE, controlled by the Austrian Habsburgs, on territories that were Protestant or on the brink of Protestantism. At the later stage, it was because of clauses in the Peace of Augsburg that did not respect territorial integrity, as that peace focused on ecclesiastical issues. Westphalia was a major departure from this system.
At a more pragmatic level, the Peace of Westphalia recognized the sovereignty of the Dutch Republic, curtailed the power of the HRE Emperor versus that of the princes, and compartmentalized religion within each princely state. And of course that huge financial settlement, which is off topic for this thread.
If I'm allowed to ask a follow up question: I've heard time and time again that the treaty if Westphalia created or reinforced the concept of nations but I've never quite understood what that means. Is it a shift away from "land which owes fealty to a king" towards "area with defined borders that has a national identity"? Because in that case I don't really see how a post Westphalian nation differs much from the Roman republic