The Thirty Years' War, a series of wars fought by European nations for various reasons, ignited in 1618 over an attempt by the king of Bohemia (the future Holy Roman emperor Ferdinand II) to impose Catholicism throughout his domains. Protestant nobles rebelled, and by the 1630s most of continental Europe was at war.
As a result of the Treaty of Westphalia, the Netherlands gained independence from Spain, Sweden gained control of the Baltic and France was acknowledged as the preeminent Western power. The power of the Holy Roman Emperor was broken and the German states were again able to determine the religion of their lands.
The principle of state sovereignty emerged as a result of the Treaty of Westphalia and serves as the basis for the modern system of nation-states.