The Thirty Years' War, fought from 1618 to 1648, devastated large parts of Central Europe through a genuinely brutal mix of religious and political conflict, drawing in most of the continent's major powers and killing a substantial share of the population in the worst-affected regions. The Peace of Westphalia that finally ended it established a genuinely foundational principle for how European states would relate to each other afterward: each state holds sovereign authority over its own territory, including its own religious affairs, without legitimate outside interference, a principle still referred to today as the Westphalian system.
The war's religious roots made settling it require a genuinely new kind of peace
The Thirty Years' War grew substantially out of religious conflict between Catholic and Protestant states, tangled together with genuine dynastic and territorial rivalries among Europe's ruling powers, and by the time negotiators sat down to end it, simply restoring the pre-war balance of power wasn't realistically available, since the underlying religious and political fractures the war had exposed needed some kind of durable, forward-looking settlement rather than a return to an already-broken prior arrangement.
Sovereign non-interference became the settlement's genuinely lasting legacy
The Peace of Westphalia's solution was to formally recognise each state's sovereign right to determine its own internal religious and political affairs, explicitly limiting other states' and the Holy Roman Empire's authority to intervene in another state's internal governance on religious or other grounds. That principle, sovereign states as the basic unit of international order, each with authority over its own territory and formally equal standing with other sovereign states, proved durable well beyond the specific conflict it was designed to settle, and it's exactly why "Westphalian" remains the standard shorthand international relations scholars still use for a state-sovereignty-based international order centuries later.
What we're still unsure about
That the Thirty Years' War caused genuinely severe devastation across Central Europe, and that the resulting peace settlement established sovereign non-interference as a formal governing principle, are well established, extensively documented historical findings. What's more genuinely a matter of ongoing scholarly debate is exactly how cleanly the Peace of Westphalia actually invented state sovereignty as a wholly new concept versus formalising and consolidating a principle that had already been emerging gradually in European political practice beforehand, and historians of international relations continue to actively debate how much genuine credit the 1648 settlement deserves as a founding moment versus how much it simply crystallised trends already well under way.
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