Hitler’s biggest mistake was a repeat of Napoleon’s, invading Russia in 1941

Hitler did not just make a mistake when he invaded the Soviet Union in June 1941. It was the first in a string of errors that continued for the duration of the conflict. German invaders didn't bring any winter gear or uniforms because they thought the war would be over quickly. They had good reason to anticipate this because up to that point, all of their land campaigns had been productive, successful, and finished swiftly. Of course, the conflict with the Soviet Union disregarded that standard, but it was not Hitler's biggest error.

His decision to declare war on the United States in December 1941 in line with how he saw the Tripartite Pact was that error. Fascinatingly, the Japanese did not return the favour and instead declared war on the Soviet Union, which was at the time at war with Germany. Despite the fact that American President Franklin D. Roosevelt did not identify Germany or Hitler in his declaration of war on Japan, Hitler nonetheless declared war. Japan was the target of American indignation rather than Germany. Roosevelt and the US had an easier time entering the war and deciding to attack Germany first thanks to Hitler.

Hitler's hasty action insured that the Allies would take control of the Atlantic shipping lanes and that his foes would be armed by the American industrial strength. The Germans then faced the biggest industrial economy in the world while already being in a death grip with the Soviet Union. A protracted and deadly war was promised by his attack on the Soviet Union. He was certain to lose it because he declared war on the United States.