The dominant images of the First World War are of a land war fought in the trenches of north-western Europe. Yet the war was ‘as much a war of competing blockades … as of competing armies’.1 These blockades aimed at interdicting enemy powers’ seaborne trade and the shipping that carried it, which the ‘first wave of globalization’ had placed at the heart of the economies of Europe and North America.2 No belligerent power was more dependent on trade and shipping than Great Britain, and it would therefore be difficult to overstate their importance to the British and Allied war effort. Moreover, shipping, ‘the world’s key industry’, as one recent study has termed it, was the one major British industry situated on the front line, directly and constantly exposed to enemy action.3