The balance of power is a venerable concept in international relations theory, but it is plagued by ambiguities about what the concept means and what the theory purports to explain, and the key proposition that states balance against concentrations of power or hegemonic threats is rarely if ever subjected to systematic empirical test. We argue that despite these ambiguities and disagreements, there is one proposition that nearly all balance of power theorists and their critics as well would accept as an accurate reflection of the theory and that provides the basis for a "most-likely" test of the theory: great powers have balanced against extreme concentrations of land-based military power in Europe, concentrations that have created the potential for hegemony over the continent.We develop and test several hypotheses linking military concentration, capability changes, and alliance responses for the European system from 1495-1999 using the Rasler and Thompson (1994) data on army concentrations and developing a new data base of great power alliances for the last five centuries. We find that European great powers have demonstrated a strong propensity to balance against the leading state when one state acquired a third or more of the total military capabilities in the system. Lower concentrations of power have been less likely to generate balancing coalitions. Not all available great powers have joined these balancing coalitions, however, so that the size of the typical balancing coalition is smaller than balance of power theory might lead us to believe.While our overall findings provide considerable support for hypotheses about balancing against high concentrations of military power in Europe, we emphasize that this is the "most likely" case for the balancing proposition, given the longstanding great power and Eurocentric biases of the balance of power tradition in international relations theory. Consequently, our findings cannot necessarily be generalized to other systems. We are particularly skeptical as to the validity of our findings for international systems dominated by maritime or global powers.
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HEGEMONIC THREATS AND BALANCING IN EUROPE, 1495-1999The balance of power is one of the oldest and most fundamental concepts in the study of international relations. Hume regarded the balance of power as a scientific law, and Waltz argued that "if there is any distinctively political theory of international politics, balance of power is it." Although there are many variations of balance of power theory, and although there is considerable disagreement about the meaning of its key concepts and even about what the theory purports to explain (Haas, 1953;Claude, 1962), the central proposition of nearly all balance of power theories is that states tend to balance against concentrations of power or hegemonic threats. Indeed, this is one of the most widelyheld propositions in the international relations field. Even this proposition is quite ambiguous, however, and it is rarely if ever subjected to systematic empirical test. O...