Note: This is an explanation of the methodology used to compile the Thucydides's Trap Case File, part of Harvard's Thucydides's Trap Project at the Belfer Center for Science and International Affairs.

In our search for cases that reflect the judgments of the leading historical accounts, we began with broad historical summaries including William Langer (An Encyclopedia of World History), Arnold Toynbee (A Study of History), William McNeil (The Rise of the West), AJP Taylor (Struggle for Mastery), Brendan Simms (Europe: The Struggle for Supremacy), and Paul Kennedy (The Rise and Fall of the Great Powers), as well as political science reviews, including Robert Gilpin (War and Change in World Politics) and Jack Levy (War in the Modern Great Power System). Consensus judgments from these broader studies were then compared with the authoritative volume of The New Cambridge Modern History. In these histories, authors have used “rise” and “rule” in their conventional definitions, generally emphasizing rapid shifts in relative economic and military strength. Most of the cases come from post-Westphalian Europe. Of the 95 inter-state wars since 1816 listed in the Correlates of War Project dataset, 6 qualify by our criteria.

In Phase 2 of the project, now underway, we are seeking to identify additional cases—including cases of hegemonic challenges in regional competitions or among less-than-major powers. We are also assembling a catalogue of metrics of national power and searching for data to provide more quantitative indicators of “rise” and “rule.” Because this is very much a “work in progress,” additions, suggestions, criticism, and comments are encouraged.

The use of historical cases for controlled comparison is methodologically complex. Issues include criteria for selection, clarity of independent variables, clarity of dependent variables, and relative weight of similarities vs. differences among cases. For a thoughtful discussion of these issues, we recommend Alexander George and Andrew Bennett, Case Studies and Theory Development in the Social Sciences; Steven Van Evera, Guide to Methods for Students of Political Science; Gary King, Robert Keohane, and Sidney Verba, Designing Social Inquiry: Scientific Inference in Qualitative Research; and the Oxford Handbook of Political Methodology.

In this project we have attempted to include all instances since 1500 in which a major ruling power is challenged by a rising power. In technical terms, we sought not a representative sample but the entire universe of cases. Therefore, as the Oxford Handbook of Political Methodology notes: “Insofar as comparative-historical researchers select what can be considered the entire universe of cases, standard issues of selection bias do not arise.”

In social science terminology, the independent variable in this study is a rapid shift in the balance of power (correlation of forces) between a major ruling power and a rising rival that could displace it. Dominance/primacy/leadership can be in a geographical area (e.g., continental Europe in the 16thcentury for the Hapsburgs) or in a particular domain (e.g., Britain’s control of the seas in the 19thcentury). The dependent variable in this inquiry is war, defined according to the standard criteria in the Correlates of War Project as military conflict causing a minimum of 1,000 fatalities per year.

A larger methodological problem for this inquiry is the absence of established metrics for assessing “national power” or the “balance of power.” These terms are regularly used in histories and political science analyses, as well as policy discussions. Yet there are no agreed-upon metrics of national power. Economic capability reflected in GDP provides a substructure for national power. But asking whether state A can cause state B to take actions that it would not otherwise have taken (or forgo actions that it would otherwise have chosen) requires combining judgments about a number of additional factors, including scientific, technical, and organizational infrastructure, and the ability of each state’s government to translate these factors into actions. In such assessments, it is not possible to avoid also making judgments about culture or propensities (for innovation, sacrifice, strategy, or fighting), the authority and competence of governments, ideology (that attracts or repels), and even statecraft.

In addition to objective metrics about one state’s rise relative to another, an important factor in our analysis is subjective perception. For Thucydides, it was not just the rise of Athens, but “the fear that this instilled in Sparta” that led to war. Thus, as we are assembling yardsticks of national power and the best historical data relevant to these metrics, we are also exploring ways to assess how objective changes are reflected in subjective perceptions of power.