More than most issues surrounding the American Mafia, the history of the Castellammare War is contestable at both theoretical and empirical levels. As the alleged pivotal event in the creation of the contemporary structure of the US Mafia or Cosa Nostra, it is of obvious importance as a topic of historical investigation. But a survey of published works on the War and its consequences reveals confusion, inaccuracies, erroneous assumptions and missing information. This is the first major systematic attempt to explore the War and its consequences made since the 1970s. Aside from adding substantially to the stock of knowledge of the War and its participants, debates on the War are critically evaluated, using original source materials where possible. The Castellammare War did not have the ramifications assumed, when placed either in a broader context or from the vantage point of internal American Mafia dynamics.