When September 1939 to May 1945

Where Atlantic Ocean, North Sea, Irish Sea, Labrador Sea, Gulf of St. Lawrence, Caribbean Sea, Gulf of Mexico, Outer Banks (U.S. coast), and Arctic Ocean

Who U-boats of the Kriegsmarine (German Navy) under Rear (later Grand) Admiral Karl Doenitz (1891–1980) and aircraft of the Luftwaffe (German Air Force) under Air Marshal Hermann Goering (1893–1946) against Allied merchant shipping between North America and the British Isles and the Soviet Union. Convoys of merchant­men were protected for the most part by British and Canadian navies and air forces aided by U.S. cruisers, destroyers, submarines, and aircraft beginning on September 13, 1941. At the outbreak of war, Germany had 57 U‑boats, of which only 22 were ocean-going types, and two surface raiders in the Atlantic. (German U-boats were joined by 27 sub­marines of the Italian Royal Navy (Regia Marina) after Italy joined the war on the side of Nazi Germany on June 10, 1940.) Against this force Britain under Admiral Sir Charles Forbes (1880–1960) could deploy 12 battle­ships and battle­cruisers, 6 aircraft carriers, 58 cruisers, over 200 destroyers, and 69 submarines.

Why The Germans applied the maritime war of attrition against Great Britain that had failed them in World War I. The strategy, which was pushed by Doenitz, was based on the belief that submarine-inflicted losses on in-bound shipments of food, fuel, and weapons could by themselves be “war decisive,” meaning it could force the island’s inhabitants into surrender. When that failed to occur, U‑boats sought to prevent the build-up of Allied supplies and equipment in the British Isles in preparation for the invasion of occupied Europe (Operation Overlord).

What The Battle of the Atlantic was the longest continuous military campaign in World War II. At its core was the Allies’ destruction of Germany’s undersea naval capability, not so much the Kriegs­marine’s struggle to blockade and isolate the British Isles, though most published histories and other media have directed the public’s attention to that aspect of the naval struggle. British Prime Minister Winston Churchill famously said that the U‑boat campaign was “the only thing that ever really frightened me during the war.” Churchill’s adversary Hitler hoped that the inter­diction of German imports (oil, iron ore, etc.) by the British, the U.S., and Canadian navies would be ren­dered point­less by German con­quests in Eastern Europe and the Near East. Greatly deficient in natural resources prior to the war, Germany would soon become self-sufficient and need no longer fear the Allied blockade or the destruction of its navy, was the thinking.

At its height from mid-1940 to the end of 1943 the Allied-Axis cat-and-mouse game to control the sea-lanes saw German U‑boats as success­ful hunters, cunning and lethal in their undersea flotillas, or Rudel (wolf packs). Throughout much of 1941, the Kriegsmarine in the North Atlantic sank 13 mer­chant vessels for every U-boat lost. After August 1942 the numerical tables began turning and U‑boats became the hunted. In May 1943 (Black May) the Kriegs­marine lost 41 sub­marines out of 118 that were at sea. On May 24, 1943, Doenitz withdrew U‑boats from the main convoy routes and abruptly ended what U‑boat crews had referred to as the “Happy Times” (die glueck­liche Zeit). Later in the year a further 39 U‑boats were destroyed after Doenitz restarted another offensive. This reversal of fortunes was perhaps the most devas­tating defeat inflicted on any German armed service and led the way to the liberation of Europe.

Outcome The German submarine war against the Allied nations failed to achieve its objectives. But in the attempt U‑boats sank 3,500 merchant, passenger, and troop ships and 175 warships, while the Allies sank nearly 800 U‑boats, or 80 percent of the U‑boat fleet—in effect, destroying the German sub­marine service. Tens of thou­sands of innocent passengers, service personnel, merchant sea­men, and sub­mariners died: 85,000 from Allied nations and 19,000 U‑boat crewmen. The hush-hush Allied penetration of the German naval Enigma codes played an import­ant role in winning the Atlantic naval battle, as did Allied airborne, surface ship, and sub­marine technical advances and offen­sive tactics from 1943 onwards. Radar, sonar, depth charges, and carrier-based and long-range shore-based aircraft led to U‑boats being sunk faster than German naval yards could replace them. Also, American ship construction outpaced German sinkings nearly five to one.

Battle of the Atlantic, 1939–1945