The invasion of the Soviet Union
Besides the military failings of Germany invading the Soviet Union, stoutly supported by the forces of their ally Romania, the attack was a mistake on several other levels. By June 1941 Germany was effectively blockaded by the British Navy, and the receipt of badly needed war materials was required to come from continental Europe. Since 1940 the Soviet Union had been supplying many of these, with Stalin using German dependence on his exports as leverage to negotiate certain areas in the region of the Persian Gulf as spheres of Soviet influence. Germany received iron ore, grains, oil, mineral oil, and other needed materials from the Soviets.
When Stalin increased his territorial demands Hitler decided to initiate his invasion, which led to early success, before the sheer weight of the Soviet armies ground the Germans to dust. His Japanese allies ignored the Pact of Steel and the Comintern Pact which they had signed, and maintained formal neutrality with the Soviets. Had the Japanese honored their agreement and invaded the Soviet Union, as well as sending their fleet into the Indian Ocean and the Persian Gulf, the conduct of the war and the post-war world would have been significantly different. But by June of 1941, Japanese preparations to attack the United States in the Pacific were well underway.