Alliance Behavior in Balance of Power Systems: Applying a Poisson Model to Nineteenth-Century Europe

This paper is a partial systematic test of Morton A. Kaplan's “theory” of alliance behavior in balance of power international systems first proposed in his well-known System and Process in International Politics (1957). Three hypotheses are inferred from Kaplan's writings predicting that in a stable balance of power system, (a) alliances will occur randomly with respect to time; (b) the time intervals between alliances will also be randomly distributed; and (c) a decline in systemic alliance formation rates precedes system changing events, such as general war. We check these hypotheses by applying probability theory, specifically a Poisson model, to the analysis of new data on fifty-five alliances among the five major European powers during the period 1814–1914. Because our research questions are so general, our findings should not be regarded as definitive; however, the data very strongly support our hypotheses. We conclude that Kaplan's verbal model of a balance of power international system has had its credibility enhanced as a result of this paper.