Some countries join interstate wars well after the war has begun, waiting months and often years, and thus changing their beliefs about the wisdom of entering a war. This volume examines why this might be so, focusing on unforeseen events in wars which cause neutral players to update their expectations about the trajectory of the war, therefore explaining why some wars spread while others do not. The author uses a combination of case studies and statistical analysis to test this theory: the Crimean War, the Franco-Prussian War, World War I, and a study of the spread of war since World War II. Designed for courses on and research into war and other international security issues, this book is a must read.