This chapter compares British preparations for economic warfare at sea before the First World War, and its practice between 1914-18. These topics are underexplored and misconstrued. Historians agree that economic warfare shaped the First World War, but neither its application nor its effect have been studied thoroughly. 1 Economic warfare aims to weaken an enemy’s economy while strengthening your own. It takes many forms, such as denying the enemy access to a commodity, making it pay a premium for the purchase, or by shattering the structure of an economy. The central means for economic warfare between 1914-18, maritime blockade, sought to prevent an enemy from exporting or importing goods by sea, denying it the ability to maintain markets and to garner foreign currency, or to acquire raw materials which it did not produce at home, so to create social and economic disruption, and to weaken its ability to produce military forces and equipment. 2 These aims were ambitious, and so were the means. During the Great War, they led Britain to intervene in the economy of every neutral country, and to struggles with most of the world’s firms. In order to blockade Germany, Britain needed to regulate imports into its neutral neighbours, like the Netherlands, Norway, Denmark and Sweden. Britain also had to control exports of raw materials and manufactured goods from distant neutral polities, most notably the United States, but also Spain, and many countries and colonies in Latin America, Africa and Asia. These needs forced Britain into complex arrangements with foreign governments and firms, which often had political power of their own.