The Battle of the Atlantic was the longest continuous military campaign of WWII, lasting from the first day of the war until Germany's surrender. It was a struggle for control of the sea lanes; Britain required over a million tons of imported material every week to survive. German U-boats, organized into 'Wolf Packs,' nearly strangled the UK by sinking thousands of merchant ships. The tide turned in 1943 due to a combination of Enigma code-breaking (Ultra), long-range aircraft, and the use of 'Hedgehog' anti-submarine mortars. Winston Churchill later wrote, 'The only thing that ever really frightened me during the war was the U-boat peril.'