Don't know about the disputes with Korea and China, but didn't the then Soviet Union just grab those 4 Kuril islands at the end of WWII after cynically declaring war on Japan at the last minute ?
I have heard even the Russian residents on those islands would prefer to be part of Japan as evidently they are totally neglected by Russia economically. Japanese sovereingty would bring a flood of services and development.
If you you describe the actions of Japanese as
"Nipponese tom foolery" then quite frankly to be fair you would have to describe Korean behaviour as
"utter insanity"
I have seen how the Japanese and Koreans relate to each other close up, and by and large they detest each other with a passion!
If Japan really wanted to stir up South Korea they could just grant sovereingty of the disputed ROCKS to the DPRK!
