In July 1936 a military revolt brought civil war to Spain; it erupted into a bitter political contest and a brutal land war which have been well covered by historians – a less appreciated dimension of the conflict was the crucial role played by the navies of both sides. This new book describes and explores the naval operations that played out over thirty-eight months and spread well beyond the seas of Spain. Spanish guns thundered off the English coast and as far south as the Bay of Guinea. Submarines lurked in the Aegean, waiting to torpedo ships bringing supplies from the Black Sea. To the west, operations extended into the Atlantic as warships searched for blockade runners and moved between the Bay of Biscay and the Mediterranean. The book begins by describing of the Spanish navy pre-war, then covers the generals’ revolt of July 1936 and its effect in the fleet. Subsequent chapters deal with the naval war as it unfolded. It describes Franco’s attempts to move the battle-hardened Spanish troops in Africa to the Spanish mainland, then shifts focus to the northern Spanish coast, where an isolated Republican enclave depended on the sea to receive weapons, food, and fuel. The author then pursues a split narrative, interweaving developments in the north and the Mediterranean and following the war to its conclusion. Significant actions fought between the two navies are all covered in detail. In addition, the book describes the crucial roles played by the British, French, German, and Italian navies in the Spanish struggle. Throughout, the author challenges the common view that the Republican navy was incompetent and inert. Based on primary as well as secondary sources, in both the English and Spanish languages, this book sheds genuinely new light on the Spanish navies of this civil war era and brings into focus their significance in the broader struggle.