These resources were of immense importance to Hitler and the German war effort. Of the three million tons of oil Germany consumed per year, 85 percent was imported, mainly from the United States, Venezuela, and Iran. When war broke out in September 1939, the British naval blockade cut Germany off from the Americas and the Middle East, leaving the country reliant on oil-rich European countries such as Romania to supply the resource. An indication of German reliance on Romania is evident from its oil consumption; in 1938, just one-third of the 7,500,000 tons consumed by Germany came from domestic stocks. Oil had always been Germany's Achilles heel, and by the end of 1941, Hitler had nearly exhausted Germany's reserves, which left him with only two significant sources of oil, the country's own synthetic production and the Romanian oilfields, with the latter supplying 75% of Germany's oil imports in 1941. Aware of his declining oil resources, and fearful of enemy air attacks on Romania (Germany's main source of crude oil), Hitler's strategy was increasingly driven by the need to protect Romania and acquire new resources, essential if he wanted to continue waging a prolonged war against a growing list of enemies. In late 1941, the Romanians warned Hitler that their stocks were exhausted and they were unable to meet German demands. For these reasons, the Soviet oilfields were extremely important to Germany's industry and armed forces as the war became global, the power of the Allies grew, and shortages started to occur in Axis resources.