PrefaceThe First World War has been the most studied war of any in human history. In part this has been because of its impact and in part because of the complicated and rich theoretical questions it raised. It would seem that little could be said that has not been said before. This is hardly the case, as new evidence still comes to light and new methodologies arise that have not yet been applied. This forum hopes to add to our insights about the First World War by doing three things. First, it utilizes a methodology that has been successful in other areas of social science-network analysis. Second, it applies the methodology to a question that has been somewhat understudied with regard to the First World War-the diffusion of war. Third, it revisits a persistent theoretical question within international relations-the relationship between systemic conditions and foreign policy decision making. The forum also does something that is often advocated and occasionally done, but not as often as it should be-generate a dialogue between quantitative political scientists and diplomatic historians.Although network analysis has a long history in sociology and has been applied extensively to study terrorism, albeit mostly in a descriptive vein, it is only beginning to be used within international relations (e.g., Maoz 2011). We believe that social network analysis is particularly relevant to the question of the diffusion of war. Networks map the linkages-how strong and dense they arebetween actors. As such, they provide ex ante a map of how war might spread once it breaks out. In Essay 1 of the Forum (Vasquez et al. 2011) we present a network analysis of the main diffusion processes that we think were at work in the spread of the First World War. Given the interdisciplinary audience, we do this in a relatively parsimonious and nontechnical manner while still presenting the basic findings and an analysis of the key networks.The question of war diffusion has been studied statistically within international relations, although it has not received the kind of attention devoted to war onset. Yet despite the extensive treatment of the First World War within political science and history, the question of why the two-party war that broke out between Austria-Hungary and Serbia in 1914 spread has spawned comparatively little quantitative data analysis (cf Choucri and North 1975). Perhaps because the role of alliances in spreading the war seems so obvious, scholars have not