THE FIRST, MOST evenly matched, and longest theater of naval battle in World War II was in the North Atlantic and, on occasion, the North and Baltic Seas. It was fought largely by U-boats and a few surface ships of the Kriegsmarine against British and North American convoys, escort warships, and long-range aircraft. The so-called Battle of the Atlantic began as a partial replay of the German U-boat effort in World War I. Once again, German naval strategists thought they might starve Britain into submission, a nation that now needed fifty-five million tons annually of imported food and natural resources to survive. Barring that, the planners of the U-boat war dreamed of disrupting the Allied blockade of German ports, or at least obtaining naval superiority near the shores of occupied Europe, thereby precluding any future Allied effort to stage amphibious landings.
Despite revolutionary breakthroughs in German nautical communications and engineering during the interwar period, what had doomed the Imperial German U-boat effort of World War I would mostly do the same to the U-boat fleet of the Third Reich. Once more, the Allies—again with the belated entry of the United States—would produce far more merchant ships than the U-boats could sink, and thus win the “tonnage war.” Once more, the Allies would develop new antisubmarine technologies and countermeasures faster and more efficiently than the Germans could improve submarines. Whereas the Imperial German U-boat fleet destroyed nearly thirteen million tons of Allied shipping by November 1918 at the cost of 178 submarines and five thousand crewmen lost in combat, the Third Reich, at war for six, not four, years, would do little better (14 million tons) while losing over five times as many crews (approximately 33,000 submariners) and over four times as many U-boats (781).1
The Battle of the Atlantic was fought over six years, from the first days of the war in 1939 to mid-1943, with residual sparring until the war’s end in spring 1945. Along with the warships of its Commonwealth allies, Britain’s fleet from mid-1940 to December 1941 was the only credible Allied navy still fighting the Kriegsmarine. The British navy tried to blockade Germany and its territories in occupied Europe as well as to starve out its armies. But such a time-tested strategy initially assumed that Germany would fight largely along its borders as it had in World War I, with its maritime access traditionally limited to the North and Baltic Seas and without access to the Atlantic. Neither assumption proved true in World War II.
Two factors are constant throughout naval history. First, the location and security of bases greatly determine the effectiveness of forwardly deployed fleets. Second, the security of such bases depends on the pulse of the land war, or at least on the status of ground forces in the surrounding territory that can transcend battle at sea. As war began in Europe in 1939, the British navy never imagined that within a year the Kriegsmarine would soon control, either through outright occupation or alliance, nearly all of the coasts of continental Europe. By autumn 1941 the Wehrmacht had absorbed much of European Russia. In contrast, Britain remained as vulnerable by sea as it had been in World War I, but with some substantial disadvantages this time around. Germany quickly obtained the shorelines of Norway and France as windows on the Atlantic for its fleet. Britain, with the exception of Gibraltar, soon lost all its prior friendly ports on the European Atlantic coastline. Hitler now also had the Italian Mediterranean fleet as an ally. Britain lost the French navy, the world’s fourth largest, as a partner. Suddenly, cutting off Britain from its imports should have been far easier than in World War I, and far more practicable than the British efforts to stall the Wehrmacht by restricting maritime imports into Germany.2
A number of general turning points would determine whether the Germans could cut off Britain. First was the Axis-Allied race to create the larger navy. More specifically, the Germans sought to replace lost U-boats and crews faster than the British and Americans could build new merchant ships and convoy escort ships. The tonnage war depended not just on relative economic strength but also on the exposure of shipping yards, pens, and related factories to enemy air attacks, and the respective strategic choices over allotment of key resources.
Second, each power sought to incorporate breakthroughs in ship design, communications, intelligence, armament, and detection more rapidly than did its adversary. The rather sudden emergence of credible German and Italian fleets by 1939 was not necessarily an accurate barometer of what would follow, given that the Allies had not been updating commensurately their own larger navies in the decade prior to the outbreak of the war. Germany and Japan eventually produced some highly innovative new nautical technologies—ranging from sophisticated torpedoes, nocturnal optics, the Type XXI U-boat, and mines, to the snorkel and complex cryptology—but they rarely shared enough of their scientific advancements in submarines. They were ill-prepared to rush new breakthroughs into mass production, at least in the brilliant fashion of the British. And they lacked the capital and manpower of the Allies’ counterintelligence and scientific agencies.3