This paper examines various legal, economic, and technological aspects of economic warfare—particularly naval blockades. It begins with a summary of the rules of international law that most developed countries “agreed” should govern “legal” blockades, and the degree to which they are enforceable. The focus is then narrowed to a particular example of a blockade—the varying success of the German submarine blockade of the British Isles and the Eastern Coast of the United States during World War II. The basis for the early German success is described, as are the contribution of the varying technological (radar, Huff-Duff, the hunter-killer groups) and organizational (convoys and command) innovations in the subsequent allied success. The role of the link between the scientific and military community and the increased ability of the Americans to produce shipping are also discussed as factors essential to successfully ending the submarine blockade. Finally, the paper raised questions about the possibility of enforcing “legal rules” during a “total” war and about the role, pro and con, of individuals in structuring blockades and countermeasures.