Short answers, pulled from the story.
Polarity in international relations refers to the distribution of power among states in the international system. Political scientists distinguish three main types: unipolarity, where one state dominates; bipolarity, where two states hold preponderant power; and multipolarity, where three or more states share significant power.
The Cold War from 1947 to 1991, when the United States and the Soviet Union were the world's two superpowers, is the most recent example of bipolarity. Earlier examples include the rivalry between Rome and Iran from 224 to 628, and the competition between Great Britain and France across much of the eighteenth century.
Scholars disagree sharply on this question. Kenneth Waltz and John Mearsheimer argued that bipolarity tends to produce relatively high stability. John Ikenberry and William Wohlforth argued for the stabilizing impact of unipolarity. Karl Deutsch and J. David Singer argued that multipolarity was the most stable structure.
According to John Ikenberry, the United States practiced strategic restraint after World War II, allowing weaker states to participate in shaping the postwar international order rather than exploiting its full power advantage. This convinced other states that the U.S. sought cooperation rather than domination, making the American-led order more durable.
The Correlates of War project uses a systemic concentration of power formula developed by J. David Singer and colleagues in 1972. The formula produces a score from near zero (power spread evenly across many states) to near one (power concentrated in few hands). A score between 0.4 and 0.5 typically indicates unipolarity; a score between 0.2 and 0.4 typically indicates bipolarity or multipolarity.
As of recent scholarship, some analysts, including Randall Schweller in a 2026 book, characterize the current system as one of unbalanced bipolarity with the U.S. and China as the dominant powers. William Wohlforth and Stephen Brooks argued in 2023 that the United States remains the unipole but with weakened power, describing the world as neither bipolar nor multipolar.