To add to Safetalker's comments,

The Japanese also ran biological warfare experimental centers in China before and during WWII. There are high rates of some very rare diseases to this day because of it. They killed a high percentage of captured Americans in the death march in the Philipines. One estimate suggested that 1/20 (5%) of the filipino population was killed by the Japanese during WWII. The Japanese believed that their Emperor was decended from a sea god and they lived by a warrior code for centuries where the Emperor's word was final.

On the other end of the spectrum, we nuked Hiroshima and Nagasaki and left them with long term radiation to end WWII. We fire bombed Tokyo to the ground. We killed so many pilots that they resorted to kamakazi tactics with untrained pilots in an attempt to respond. We incarcerated 113,000 Japanese US-immigants and their US citizen families in the US and wiped out the finances of many of the rest. We removed their ability to rebuild a military and have had a strong military presence in Japan for nearly 70 years. 1000 Japanese were put to death for war crimes and 4400 others were incarcerated.

China, Korea and the Philipines may still have hatred, but the Japanese have been "tamed" for lack of a better word. They have high rates of suicide, their economy has drifted for 25 years, and they keep trying to use Keynesian economic theory to stimulate their economy which is the same as bludgeoning them economically.

Launching a Nuke on Japan now would only END WWIII badly for everyone.