Daniel Philpott, Revolution in Sovereignty (2001), Chapter 5.

Summary: The treaty of Westphalia marks the origin of the sovereign states system. While states existed before hand, the treaty in 1648 marked the moment where states, and the state system, prevailed over the old order premised on the religious community.  Three aspects of the treaty proved to be durable and self-sustaining in the persistence of the state system: legitimacy, criteria for entry, and rules for relating to others.  These three faces of sovereignty acted not only to define the new system, but also to maintain it.
Note:
Argument: Westphalia was the origin of the sovereign states system.
– Before 1648, as long as the 30 years war was still smoldering, significant features of political authority in Europe were incompatible with sovereign statehood. In Philpott’s view, Westphalia was the culmination, not the creation, of the modern state system.
Absent Sovereignty: The Middle Ages
– Society was united in the body of Christ
– Natural law prevailed, but no sovereignty. There was instead moral/legal unity.
From Middle Ages to Westphalia
– Slow shedding of eclecticism and the popping up of sovereign states
– The thirty years war = religious war against the existence of rival religions next door
– The end of this war proscribed such intervention with the peace of Westphalia
The Novel Significance of Westphalia
– Three key European disputes were settled in 1648
– Participants all considered themselves part of a common society, but participated as separate political entities.
— There was a recognition of themselves as a society of sovereign states
– The triumph of the state system over the Empire – this was not codified in the treaties, but states became the chief form of constitutional authority.
– The leitmotif in the negotiations was state autonomy
– Immediately after Westphalia, sovereignty emerged most saliently
– Princes consented to internal pluralism, to respect the rights of dissenters to worship, and to refrain from converting one another’s subjects.
The Formative Legacy of Westphalia
– It defined the holders of sovereignty and their prerogatives
– Three faces of sovereignty proved durable:
– First face (Legitimacy)  – the state became the legitimate polity within Europe, while the Holy Roman Empire virtually lost its sovereign prerogatives. States deepened their legitimacy through subsequent laws and self-reference to the concept. There was also global replication.
– Second face (Criteria) – There were strict criteria of what kinds of organizations could become, and retain, sovereign state status. This also proved to be a durable feature.
– Third face (Relations with Others) – Rules were established to allow for relations between states, these also proved to be durable.
There was a distinct contrast between the world prior to Westphalia and the one that emerged and has persisted since.