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The Battle of the Atlantic (1939-1945), the longest-running campaign of World War II, witnessed the clash of German U-boats against Allied convoys. Grand Admiral Erich Raeder's strategy of unrestricted submarine warfare aimed to cut off Britain's vital supply lines and force its surrender. While initially successful, the Allies' advancements in radar, code-breaking, and long-range aircraft gradually turned the tide against the German Kriegsmarine.