The Spanish Civil War of 1936-1939 underlined the importance of the sea as the supply route to both General Franco’s insurgents and the Spanish Republic. There were attempted blockades by Franco as well as attacks by his Italian and German allies against legitimate neutral, largely British, merchant shipping bound for Spanish Republican ports and challenges to the Royal Navy, which was obliged to maintain a heavy presence in the area. The conflict provoked splits in British public opinion. Events at sea both created and reflected the international tensions of the latter 1930s, when the policy of appeasement of Germany and Italy dissuaded Britain from taking action against those countries’ activities in Spain, except to participate in a largely ineffective naval patrol to try to prevent the supply of war material to both sides. The book is based on original documentary sources in both Britain and Spain and is intended for the general reader as well as students and academics interested in the history of the 1930s, in naval matters and in the Spanish Civil War.