I don't remember the UK becoming a pariah state for recovering the Falklands Islands when they were conquered and occupied by Argentina.
In these scenario, Japan had attacked a sovereign state and conquered and occupied foreign territories. As an attacked country, Russia can use every means to recover its territory...
I don't believe Kouriles Islands being the home of a big or even small minority of japanese people and Japan had violate a WWII treaty peace.
Japan will be condemned in the UN, be subject to an embargo and every asians countries will applaude with both hands because a military japanese invasion occured and everybody will fear to the next unwilling candidate to the Japanese dominated "Asian Sphere of Coprosperity"...
Remember this is 1991 when Japan was the second biggest economy in the world and begin to have a kind of "victory disease" in the economic sphere...
japan never ever will attack russia, not in 1991 and not in any other time after 1939
this cannot happen... period.
so if the russian size parts of japan it will be russia that will go down, it was totally depending on foreign money (german one) in 1991... if the hardliner have sucsess with their putsch against gorbatchow, they will do anything but no war against japan.
nukes are a nogo, conventional the russian army wasn´t combat capable to fight a war with combined naval and armed forces...
the american fleet would support the japanese defence forces, the latter had been one of the best armies in the world, cause they are very advanced... so any russian try to invade are doomed, only nukes could be possible, but nukes for what?
no way, asb