Greatest blunder? OK I will give my 2 cents. The greatest blunder was giving back Japan her nation. She was nation building and had taken most of Asia. That included killing 30 million Chinese and a few million other Asian folks. She surrendered UNCONDITIONALLY and the allies treated her better than she had or would have any nation she defeated.
In what way was that a mistake? We have peace with them nowadays, haven't we? They don't threaten other nations anymore, do they? With your logic, you shouldn't have treated germany so well and they wouldn't have been one of your most loyal ally during the cold war.
To show mercy is to show one's greatness