http://ww2live.com/en/content/world-war-2-world-war-ii-battle-atlantic-online-interactive-now-available In 1939 German U-boats began stalking Allied merchant ships with the goal of cutting vital shipping lanes. In the first months of the war the Germans had limited success due to Allied control of the waters surrounding northern Europe and critical passages through them. Once the Germans seized control of Norway and France in 1940, the dynamic shifted. The Germans controlled much of the English Channel and North Sea coasts, and had easy access into the greater Atlantic. They soon adopted “wolf pack tactics”, traveling and attacking in groups rather than hunting alone. When the United States entered the war in December 1941, shipping lanes along its east coast became key target areas for the U-boats, which inflicted terrible losses. The heaviest fighting and climax of the Battle of the Atlantic occurred in late 1942 and early 1943. The advantage shifted to the Allies as they massed manpower and equipment, refined their escorted convoy system, deployed air coverage, and utilized new technologies to track and destroy U-boats. By 1944 the threat from German U-boats had been dramatically reduced, and the Allies maintained control of the waters of the Atlantic until the end of the war. Allied ships, troops and supplies sped across them uninterrupted.