States engage with one another in an environment known as the international system. All states are considered to be sovereign, and some states are more powerful than others. The system has a number of informal rules about how things should be done, but these rules are not binding. International relations have existed as long as states themselves. But the modern international system under which we live today is only a few centuries old. Significant events have marked the milestones in the development of the international system.
In 1648, the Peace of Westphalia, which ended the Thirty Years’ War between Catholic states and Protestant states in western and central Europe, established our modern international system. It declared that the sovereign leader of each nation-state could do as she or he wished within its borders and established the state as the main actor in global politics. From that point forward, the international system has consisted primarily of relations among nation-states.
In the seventeenth and eighteenth centuries, the nation-state emerged as the dominant political unit of the international system. A series of powerful states dominated Europe, with the great powers rising and falling. Weaker states often banded together to prevent the dominant power from becoming too strong, a practice known as preserving the balance of power. Frequent wars and economic competition marked this era. Some nations—notably France and England—were powerful through most of the modern age, but some—such as Spain and the Ottoman Empire—shrank in power over time.
The nineteenth century brought two major changes to the international system:
The problems raised by the unification of Germany contributed to World War I (1914–1918). In the aftermath of the war, the international system changed dramatically again. The major powers of Europe had suffered greatly, whereas the United States began to come out of its isolation and transform into a global power. At the same time, the end of the Ottoman and Austro-Hungarian empires created a series of new nations, and the rise of communism in Russia presented problems for other nations. These factors contributed to the Treaty of Versailles, the rise of Nazism and communism, and World War II (1939–1945).
The end of World War II marked a decisive shift in the global system. After the war, only two great world powers remained: the United States and the Soviet Union. Although some other important states existed, almost all states were understood within the context of their relations with the two superpowers. This global system was called bipolar because the system centered on two great powers.