The Treaty of Westphalia effectively ended the Thirty Years' War (1618-1648) in Europe and laid the foundation for the sovereign state system that has dominated international relations in the modern era. Although a complex and prolonged war, the Thirty Years' War nominally began as a conflict between Catholics and Protestants which dragged in most of the Great Powers of Europe. Because of the religious nature of the war, the most important tenet from the peace to emerge was the idea that each prince had the right to determine the religion in his land. At this time, there was no German nation as we today know it. Instead, the Holy Roman Empire was a loosely governed collection of independent kingdoms and principalities located mainly where modern Germany and Austria are. Because these German lands had been the location of much of the activity of the Protestant Reformation, Lutheranism and Catholicism were in direct competition with one another. By saying that a prince had the power to determine which religion would be practiced in the land he governed, free of outside influence or force, the Treaty of Westphalia laid the foundations for the idea of complete sovereignty of the ruler over their people. This might seem like an obvious system to modern observers, but in an era of empires, protectorates and influence from outside secular and religious forces, it was radical. It would be states (and later 'nations') that would decide internal policy, not empires or the church.
Although in the modern era kingdoms and principalities gave way to constitutional and representative forms of government, the principle of sovereignty still applied. In representative democracies the people who comprised the nation became sovereign to rule over their land in the manner decided by the majority.