As described in chapter 1, one of the motivations of this project is the effort to extend knowledge about the conditions under which great powers (and the developed world more generally) fight wars, so as to determine whether that knowledge can increase our understanding of the conditions under which developing states fight wars. I suspect few would object to such a motivation, but some might not agree with the specific body of existing knowledge I use as the basis for my extension. In this book I extend and adapt power transition theory, a theory developed to account for the incidence of wars fought for control of the international system among the very strongest of states. Some might question why I would focus on any structural theory of international behavior when strategic theories have gained such popularity and offered so many insights. Others might question why I would specifically select power transition theory as the best candidate among structural theories. In order to address such potential concerns, I describe power transition theory in some depth. Having summarized the theory, I suggest it is the best candidate structural theory because it has achieved so much empirical support and offered clues about a variety of international phenomena beyond war and peace. I then address concerns that strategic theories may offer a more promising avenue for extensions such as I attempt.