Final answer:
One of Hitler's biggest mistakes was invading the Soviet Union in Operation Barbarossa. This decision weakened Germany and diverted resources from other theaters of the war.
Step-by-step explanation:
One of Hitler's biggest mistakes was his decision to invade the Soviet Union, known as Operation Barbarossa. Despite initially achieving some successes, the German army was ultimately unable to conquer Russia and faced a harsh winter and fierce resistance. This mistake weakened Germany's military and resources, diverting valuable forces and supplies from other theaters of the war.
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