"The only thing that ever really frightened me during the war was the U-boat peril," wrote Winston Churchill in his monumental history of World War Two. Churchill's fears were well-placed-the casualty rate in the Atlantic was higher than in any other theater of the entire war. The enemy was always and constantly there and waiting, lying just over the horizon or lurking beneath the waves. In many ways, the Atlantic shipping lanes, where U-boats preyed on American ships, were the true front of the war.
England's very survival depended on assistance from the United States, much of which was...