I'd say it was zero as soon as they gave the US such a large piece of internal propaganda (the attack on Pearl Harbor). Without that, and without a declaration

I'd say it was zero as soon as they gave the US such a large piece of internal propaganda (the attack on Pearl Harbor). Without that, and without a declaration of war, Japan might have been able to ignore US forces in the Pacific. Getting the US to attack Japan in response to attacks on countries full of lesser peoples would have been a difficult proposition, and the willingness to fight on through the slog that the South Pacific became might not have been there. But after Pearl Harbor...