One aspect of system factors is that there have been different system structures at different times. Under the vast Roman Empire, Europe and the Mediterranean were unipolar, which is rare. During the 1947-91 Cold War, the system was bipolar, with two powerful nation-states (the U.S. vs. the USSR/Russia). In most periods, it was multipolar, that is, with several powerful nation-states (e.g. 1871-1945, with Britain, France, Germany, Russia, Japan and the U.S.).
Some analysts say that today we have a mixed system, with what the Chinese call one superpower (the U.S.) and many great powers. In The Paradox of Power, Joseph Nye says that since the fall of the USSR, the U.S. has overwhelming military power, but in economics and politics, the world is multipolar. Thus, the U.S. had the military power to go to war in Iraq in 2003, but did not have the political power to convince the other members of the UN Security Council to officially authorize it. In economics, the U.S. economy has decreased from about 50% of the world after the total destruction of its economic rivals in WWII to a more historically normal 22% today. The 29 nations of the European Union have a population larger than the U.S. and an economy second only to the U.S., and they do not hesitate to file World Trade Organization complaints against the U.S. or to find Microsoft or Google in violation of European anti-trust laws. China openly competes with the U.S. and will probably be the world’s largest economy in another generation. As China’s and other nations’ military power increases, we may return to a system that is multipolar in all dimensions.
The relative power of the different powers is also an important system factor. For 200 years, the Western powers had overwhelming military and economic power. Now many Asian nation-states, especially China, are rising both economically and militarily. Indeed, the biggest factor in world politics today is the rise of China. As a result, there have been many changes and adjustments. For instance, the U.S., India, Japan and Australia (the Quad) have developed closer relationships in order to counter China.
Another system factor is the degree of integration among the various players. During the Cold War, the U.S. organized the NATO alliance and the USSR organized the Warsaw Pact in Eastern Europe. Today, the major powers are all linked by numerous trade and other economic ties.