As exhibited by Bill Clinton’s quote on Bosnia, leaders have long been concerned about the possibility civil wars could spread to involve additional, often neighboring, states. Of course, the great irony of Clinton’s and other Western leaders’ concerns about the civil wars in the former Yugoslavia is that the wars never spread to include any states other than the successor states to Yugoslavia until after those very leaders decided to intervene in Bosnia and Kosovo. Still, in general the concern is valid and raises the question of why, in fact, do civil wars at times spread to involve additional states. Multiparty wars, both interstate and civil, are rare, but whether they are just outliers from bilateral wars or whether they are somehow the result of distinct causes has been debated.1 This book’s aim is to answer that question. Its central argument is that civil wars spread as a result of conscious decisions made by third parties who are reacting to information revealed by events within civil wars. Thus, such wars differ from bilateral conflicts because of how other states react to events within those wars, rather than because the wars are fundamentally different in nature.