A quarter of a century ago, three young scholars-in an unexpectedly symbiotic fashion-converged to identify and, perhaps, clarify what has come to be called the "level of analysis problem." In Man, the State, and War (1959), Waltz carefully articulated the empirical premises that are more or less inexorably associated with the several levels of analysis/ aggregation and concluded that most of the variance in the incidence of war and other major events could be accounted for by the characteristics of the international system. While Singer (1960, 1961) concurred with that theoretical judgment, he pursued the question a bit further, suggesting that each level-the individual, the national society, and the international system-had a role to play in shaping the behavior of the nations and the fortunes of their citizens. The third of the new and interdisciplinary breed of scholars central to this discussion was Kaplan (1957), who not only concluded that the international system was "subsystem dominant" in the sense that the more significant aspects of international politics were determined by the properties and behaviors of the nations rather than the properties of the system, but also went on to postulate a set of axiomatic rules that seemed to govern that behavior. Among others, those who participated actively in these exchanges were Rosenau (1969), Haas (1953), Russett (1967), Hoffmann (1965), Liska (1956), Rosecrance (1966), and Deutsch and Singer (1964).