The Peace of Westphalia was signed on October 24, 1648. It ended the Thirty Years' War, the long and terrible conflict that had devastated Germany, and it settled the religious and political questions that had set Europe on fire. The map changed that day.
The war had begun over religion. It lasted thirty years. It started in 1618 as a conflict between Catholics and Protestants inside the Holy Roman Empire, and it drew in France, Sweden, Spain, and Denmark until half the continent was fighting. Whole regions were emptied, and Germany lost a large part of its people.
The peace recognized the split. The treaties confirmed the right of rulers to choose the religion of their lands, and they gave the German princes a freedom from the emperor they had never had. The Holy Roman Empire survived in name, but its power was finished.
The principle mattered more than the details. Westphalia established that states were sovereign, answerable to no higher authority, and that their borders were not to be violated lightly. The idea became the foundation of international law and of the modern state system.
The peace is still cited today. Diplomats and scholars speak of the Westphalian system to mean a world of independent states that do not interfere in one another's affairs. A treaty signed to end a religious war became the reference point for how the world organizes itself.