There is literally nothing that the 1940s can do to offer more than a speed bump to an Empire that can churn out hundreds of plasma cannon toting battleships and thousands of six story mecha who can eat killsat blasts to the face and saw tanks in half with their eye beams. Not even getting into their mass usage of supersonic jets, guided missiles, directed energy weapons, power armor, transformers, nanotechnology, and literal combat psionics. Not to mention being able to construct mobile military cities that can house and supply massive numbers of soldiers, warships, and all their equipment to allow for extremely long-range offensive operations like invasions of the mainland of Europe and North America.
The Japanese conquer the world in weeks.
Even if the world could get out an atomic bomb fast enough before the Japanese Empire overruns all opposition (they can't), the fact that it has to be delivered by slow-moving prop-engined bombers means it'd basically be useless anyway.