From Williamson Murray "Luftwaffe 1933-1945"

Hitler had calculated that the Nazi Soviet Non Aggression Pact would mitigate the effects of an Allied blockade. Reality proved quite different. Import tonnage fell 57 percent.By January 1940 the value of imports had fallen to RM 186 million as compared to RM 472 million in Jan 1939 while import tonnage declined from 4,445,000 tons the previous year to 1,122,000 tons. Moreover, petroleum reserves declined from 2,400,000 tons at the beginning of war to 1,600,000 tons in May 1940 while gasoline supplies fell from 300,000 tons in Sept 1939 to 110,000 tons by April 1940.

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