Operation Downfall would do the trick. For the following reasons...
According to a study by William Shockley for Secretary of War Henry Stimson , Japanese casualties from the operation would've been up to 5-10 million, mostly from the involvement of civilians in the defence of the country.
Chemical weapons were to be used to prevent the American bloodbath that was Okinawa from happening again. Herbicides were also to be used against Japanese crops.
Ken Nichols, District Engineer for the Manhattan Engineer District, said that fifteen atomic bombs would've been used to support the Americans as they invaded Honshu, the main island of the Japanese archipelago. Other sources say no more than five atomic bombs would've been used during the invasion.
In other words, Japan was to be completely annihilated and it's population would've gone through hell, even more than Germany did. This experience would've been enough to cause a seething hatred of the US to be left in the Japanese psyche.