"I think he'd take the US down completely rather than give it up as a miscalculation."
Although that motive could be lurking in his mind, I think it's his ego relying on his religious zeal and his megalomania that would actually make him think he could pull it off.
Remember, this is a born again Christian who believe God chose him to lead Armageddon.
Thing is, after Iran supplying arms, then will come Russia, Pakistan, China, and finally the European countries. If the US is stupid enough to stay and fight, they'll just be sitting ducks. An arms dealers bazarre(And Bush will be too stupid to see this.)