Chapter 2 set out a theory of how oil and revolutionary governments generate international conflict. I now seek to test the theory against the empirical evidence. My strategy is to subject the hypothesized relationships to multiple tests using different kinds of evidence and methods. Subsequent chapters carry out the various tests.

This type of research design, using multiple empirical methods to test a theory, has produced some of the best research in political science. The key benefit of using both quantitative and qualitative methods is that the strengths of each method tend to compensate for the weaknesses of the other. For instance, the statistical analysis complements the qualitative case studies by revealing the relative importance of multiple variables, something that is nearly impossible to do convincingly using a small number of cases. The rigor of statistical methods also serves to guard against unintentional selection bias that can crop up relatively easily in case studies. The qualitative analyses, on the other hand, are much better at revealing the political processes and causal mechanisms that link the explanatory variables to the state’s propensity for international conflict. Historical case studies also allow us to examine the evidence of whether and how decision-makers considered alternative options and “off-the-equilibrium-path” outcomes, other than the ones they actually chose. Overall, a multi-method approach provides a parallax for theory-testing that is difficult to achieve using a single methodology. As Beth Simmons points out, “to choose one method to the exclusion of the other is like closing one eye and trying to make judgments about distance: it is easy to lose perspective.”