I think the single biggest mistake he made was declaring war on America. He should have issued a weaselly statement about supporting Japan in its struggle against Western Imperialism and left it at that.
Yup, another idiotic move that had no rhyme or reason.
It was dumb enough to take on the gigantic USSR.
But taking on the world’s largest industrial nation, well beyond the reach of his panzer divisions and air force was simply insane.