Why does warfare occur? Why do some wars expand to encompass the entire system? Can more peaceful structures be built? These have been some of the burning questions of the twentieth century. Practical politics has provided answers to these questions. Scientific analysis has found those answers wanting.

The next five chapters will carefully sift through the last twenty-five years of scientific research to piece together new answers to these questions. These answers will be suggestive, not definitive. How much confidence we can have in them depends on the success of the research that has been conducted to date. I seek to present all the relevant scientific findings as pieces of evidence that must then be examined and synthesized, before they can be used to provide answers. The analysis of war presented in this part of the book, therefore, must be seen as an exercise in theory construction. I am seeking to explain war by explaining existing findings, but since these findings are sometimes inconsistent and incomplete, a full explanation of war, particularly its onset, requires not only that I interpret the evidence, but that, at times, I go beyond the evidence and hypothesize on the basis of historical example or inductive reasoning. When I do this, and I will do it at certain critical junctures in the argument, it should be clear from the context and the absence of references to data. For this reason, as well as standard criteria of scientific rigor, the explanations offered in this part of the book must be seen as tentative until they are tested deductively.