o DECLARING WAR ON THE US - if Hitler had not declared war on the US, it is not clear the US would have declared war on Germany...imagine Hitler denouncing the Japanese and in a very public way saying it was willing to help the US defeat Japan! (imagine the effect on US public opinion particularly given the German/Italian/Irish population which was inherently anti-British)
o THE BALKANS - if the Italians had been able to take Yugoslavia by themselves, the Wehrmacht would have had many more soldiers and assets to use against the Soviets and Stalingrad could have had a very different outcome.
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I recently read a book on famous strategic mistakes throughout history, and all three of these made the list. First, Russia and the Balkans. Hitler had originally planned to launch Barbarossa earlier to avoid the Russian winter that had stymied Napoleon, but was forced to divert to Yugoslavia to save Mussolini. Oops.
And then, there's Japan. The Germans were caught as off guard as anyone else, and were much relieved the next day when Roosevelt asked for a declaration of war against only Japan. The idea of going up against a force which could "sit back for years and build up a massive force which, once unleached, would be unstoppable" was unthinkable. And apparently Hitler didn't think about it.