The name given to a succession of sea‐operations in World War II. They took place in the Atlantic, the Caribbean, and northern European waters and involved both submarine blockades and attacks on Allied shipping. German U‐boats, sometimes assisted by Italian submarines, were the main weapon of attack, but aircraft and surface raiders also participated. About 2800 Allied, mainly British, merchant ships were lost, placing the Allies in a critical situation. After summer 1943, with the introduction of better radar, the provision of long‐distance aircraft and of escort carriers, and the breaking of German codes, the situation eased, although technical innovations subsequently increased the U‐boats’ effectiveness. It was only the capture of their bases by Allied land forces that finally put an end to the threat.