The moment
The Thirty Years' War began in 1618 as a Bohemian revolt against Habsburg rule, but by 1635 Catholic France was fighting the Catholic Habsburgs, and dynastic rivalry mixed with religion throughout. The treaties of Münster and Osnabrück extended the Peace of Augsburg to Calvinists, fixed 1624 as the reference year for each territory's religion, and are often said to have laid foundations for the modern state system.