Dönitz calculated that he needed to sink 600,000 tons of British shipping a month to achieve a decisive victory, for which he required 300 U-boats in commission to sustain a third of that number in operational areas. Yet only 13 U-boats were on station in August 1940, falling to 8 in January 1941, rising to 21 the following month. This small force inflicted impressive destruction: 2 million tons of British shipping were sunk between June 1940 and March 1941. But in the same period just 72 new U-boats were delivered, far short of the number Dönitz needed. They achieved their highest rate of
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