Oil
Oil was a critical resource of WW2. It was used to produce fuel that powered every vehicle used in the war. From ships to planes, the combatants of WW2 needed oil. However, like most resources Germany lacked sufficient supplies to meet their own needs during peacetime, so during wartime, shortages would be even worse. Even with German efforts to produce synthetics yielding great results along with Germany taking over Romanian Oil fields; the German Reich was consuming about 25% more Oil then it could produce. These shortfalls were being made up for by shipments from the Soviets. The Nazi leadership, however, feared that the Soviets would one day cut off these shipments. This was another resource that Germany tried to secure when invading the Soviet Union.