Europe was thrown into balance-of-power politics when its first choice, the medieval dream of universal empire, collapsed and a host of states of more or less equal strength arose from the ashes of that ancient aspiration. When a group of states so constituted are obliged to deal with one another, there are only two possible outcomes: either one state becomes so strong that it dominates all the others and creates an empire, or no state is ever quite powerful enough to achieve that goal. In the latter case, the pretensions of the most aggressive member of the international community are kept in check by a combination of the others; in other words, by the operation of a balance of power.
The balance-of-power system did not purport to avoid crises or even wars. When working properly, it was meant to limit both the ability of states to dominate others and the scope of conflicts. Its goal was not peace so much as stability and moderation. By definition, a balance-of-power arrangement cannot satisfy every member of the international system completely; it works best when it keeps dissatisfaction below the level at which the aggrieved party will seek to overthrow the international order.
Theorists of the balance of power often leave the impression that it is the natural form of international relations. In fact, balance-of-power systems have existed only rarely in human history. The Western Hemisphere has never known one, nor has the territory of contemporary China since the end of the period of the warring states, over 2,000 years ago. For the greatest part of humanity and the longest periods of history, empire has been the typical mode of government. Empires have no interest in operating within an international system; they aspire to be the international system. Empires have no need for a balance of power. That is how the United States has conducted its foreign policy in the Americas, and China through most of its history in Asia.
In the West, the only examples of functioning balance-of-power systems were among the city-states of ancient Greece and Renaissance Italy, and the European state system which arose out of the Peace of Westphalia in 1648. The distinguishing feature of these systems was to elevate a fact of life—the existence of a number of states of substantially equal strength—into a guiding principle of world order.
Intellectually, the concept of the balance of power reflected the convictions of all the major political thinkers of the Enlightenment. In their view, the universe, including the political sphere, operated according to rational principles which balanced each other. Seemingly random acts by reasonable men would, in their totality, tend toward the common good, though the proof of this proposition was elusive in the century of almost constant conflict that followed the Thirty Years’ War.