When one state grows too powerful, other states tend to form alliances against it—a process called balancing. Balancing can be external (forming coalitions) or internal (building military power). Alliances form when states face a common threat and see mutual defense as beneficial. The strength and durability of alliances depend on the distribution of power, the intensity of the threat, and the compatibility of members' interests.
Compare alliances against Napoleon, Nazi Germany, and the Soviet Union. Why did Britain lead or join coalitions to balance each of these threats?
Balancing is not automatic—it depends on perceiving a threat, having the means to balance, and preferring balancing to bandwagoning (joining the stronger side).
Your prerequisite on the balance of power established that international stability tends to emerge when power is distributed rather than concentrated in a single dominant state. Alliance formation is the mechanism by which states actively maintain that distribution. When one state begins to accumulate disproportionate power, other states face a strategic choice: resist by joining forces, or accommodate by joining the rising power. Balancing is the resistance strategy; bandwagoning is accommodation. Classical realist theory predicts that balancing is the dominant pattern — states prioritize survival and fear domination, so they resist hegemons even when joining would offer short-term benefits.
The historical record largely supports this pattern. Napoleon's France, which by 1810 controlled most of continental Europe, faced successive coalitions — six in total — organized by Britain and the other great powers. Nazi Germany's rapid conquest of France in 1940 triggered alignment with Britain rather than capitulation by the remaining European states, and eventually drew in the Soviet Union and the United States. The Cold War was structured by the systematic alignment of most industrialized states against Soviet power. In each case, states that shared no obvious common interests — monarchies, republics, colonial empires — submerged their differences to balance against a more immediate threat.
External balancing (forming alliances) and internal balancing (building military capability) are the two main instruments. External balancing is faster and cheaper — it multiplies power by adding allied capabilities without the long lead times of military investment. But it comes with costs: commitments, entanglement in allies' conflicts, and dependence on partners whose interests may diverge. Internal balancing is slower and more expensive but avoids these risks. Most states use both, calibrating the mix to their strategic circumstances.
The key variable that determines whether balancing occurs is threat perception, not power alone. Stephen Walt's refinement of the theory holds that states balance against threats, not just power — and threat is a function of aggregate power, geographic proximity, offensive capability, and perceived aggressive intentions. This explains apparent anomalies: Canada does not balance against the United States despite the enormous power asymmetry, because the US is not perceived as threatening. Conversely, small states sometimes balance against a regional neighbor with modest power if that neighbor is perceived as expansionist. The implication is that states can reduce others' incentives to balance against them by signaling restraint and limiting their own offensive reach — a lesson with direct implications for great-power competition today.
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