Well-known as a leading voice on military AI (his Army of None introduced a large readership to the debates surrounding autonomous weapon systems), in Four Battlegrounds, Scharre casts the net much wider by examining how AI is likely to alter the international security framework that has endured for the last fifty or so years. This volume provides a nuanced overview of how AI is disrupting settled security practices regarding privacy, democratic process, civil-military relations, military logistics, tactics, and strategy. The ‘four battlegrounds’ refer to the principal arenas where international rivalries over AI are playing out: the procurement of massive data sets to train AI learning algorithms, the manufacture of computer chips and other computational resources, attracting top AI talent, and devising nationwide institutional frameworks to foster AI innovation. While first and foremost about power relations (especially the US-China rivalry), Scharre also provides a running commentary on the attendant ethical issues. The book is structured around short chapters that provide vignettes on leading-edge issues such as AI empowerment of the surveillance state, the challenges faced by innovative defense company start-ups, deep-fakes and political subversion, data ‘the new oil’, the dilemmas of sharing research across national borders, the advantages of centralized versus ‘open’ approaches to AI innovation, and how to engineer AI systems safely in a context of acute geo-political rivalry where competition can lead to a ‘race to the bottom’. In sum, this book provides a balanced and well-documented treatment (accessible to a broad readership) of a theme that is sure to become increasingly relevant in the years to come.