One of the most successful elements of the German Kriegsmarine was the U-boat arm.

. Organized by the master tactician Karl Doenitz, the U-boat fleet would be the one part of the German military that would truly frighten Prime Minister Winston Churchill (as mentioned in his concise history of WW2).

With Hitler's call for rearmament, Doenitz believed he could decimate the sea lines of communication and trade that were so vital to the British Empire if he had 300 boats.

But when war broke out in September of 1939, Germany was nowhere near Doenitz's goal of 300 submarines. Instead, there were approximately 45 operational boats out of a total of 57. Of these, only 22 were capable of service in the rough seas of the Atlantic. Until 1943, when Hitler finally called for a cessation of all large surface fleet construction, only 5% of German steel production was dedicated to the U-boat mission that Doenitz had to carry out. Early reliabilty problems with torpedoes and lack of good air support also hampered initial U-boat efforts.

Still, the early years of the war were to yield excellent results and were thus named the "Happy Time." Guenther Prein's attack on the Scapa Flow naval base and the sinking of the British battleship Royal Oak was a major propoganda victory for Germany during this period as well. And with the instituting of "Wolfpack" tactics, British shipping was being sunk at an alarming rate.

Into 1941, Great Britain had finally gained a grip on the situation with a combination of convoying, radar, ASDIC detection, and better escort tactics. Additionally, capture of code books and an Enigma encoding machine allowed the British to read German radio messages, and U-boats were no longer able to operate in secrecy.

With US entry into the war in December of 1941, U-boat captains like Erich Topp and Reinhard Hardegen headed for the American east coast and the second Happy Time began. In the first six months of 1942, nearly 400 American ships were sunk at a cost of over 5000 lives lost--the greatest disaster at sea ever suffered by the US. The addition of a fourth rotor to German Enigma machines also put the Allies in the dark again as to U-boat movements. By the end of 1942/early 1943, Great Britain was desperately short of fuel and supplies, and it looked very much like Germany would win the Battle of the Atlantic, and quite likely the war along with it.

The final turning point came, however, in what the Germans called "Black May" (1943). In this month alone, more U-boats were sunk than was Allied merchant shipping. What helped bring this about was the breaking of the new German code, improved radar, high frequency direction finders (HF-DF), and overwhelming Allied ship production. Doenitz attempted a major push back into the Atlantic in the fall of 1943, with several new advances of his own (such as acoustic torpedoes and the snorkel "breathing" apparatus), but the offensive was short-lived. U-boats would continue to patrol up to the very last day of the war, but 1944 and 1945 went badly for the Ubootwaffe overall.

In the end, about 800 of 1100 U-boats constructed had been sunk. 30,000 of 40,000 U-boat men who set out on patrol never returned home. In the history of warfare, no combat force has ever sustained such a loss rate and still maintained its spirit and sense of duty. The U-boats themselves sank nearly 3000 Allied vessels, for over 14 million tons of shipping. The Battle of the Atlantic had been the single longest and costliest battle of the Second World War.