Okay to start off,this is pure speculation,im well aware that such war is extremely unlikely so im not really looking for people saying "oh it will never happen" this is not ment to disrespect anyone or ether one of these great countries its just a question on strategic points of view,what ether side would do to defend or attack what resources or technology would be the turning point of the conflict,estimated amount of casualties on ether side and the wars aftermath. Now for some time after the fall of soviet union Russian forces faced cut budget,corruption and there forces suffered from lack of maintenance but now after Putin and then Medvedevs reign they seem to be returning on there place of worlds superpower and the last conflict they had with Georgia that usa backed and trained proofed they are prepared and capable of an invasion if there interests are being hurt.Similarly the Japan is trying to reclaim the islands that belonged to them before the end of world war 2 Kurils islands that have Russian citizens living there,while so far very diplomatic hypothetically they could occupy the islands and start the war,unlike the Georgia who mostly had left over soviet tech no navy and very limited trained forces,Japan ranks from what I read third most powerful navy,there air-force and radars are also superb so I don't believe the war will end as quickly and as easy for Russians. Nukes,obviously Russians have alot and there advances in ballistics are second to none,tho I personaly don't believe they will use them in war with Japan in fear of international backlash,and fallout they will create too close to there own borders and china borders not to mention Japan has none and it wouldn't be justified,but if anyone thinks otherwise id love to hear your thoughts. Allies,Japan being one of the key pieces of world economy even now after the terrible crisis due to tsunami and reactor accident they endured,so they would most likely be backed by united states but will united states commit military force or just supplies?in case with Georgia also an ally of usa they didn't do much to help at all,while European union will most likely back Russians or stay neutral due to there dependency on Russia on natural resources and economy crisis in Greece taking immense resources there barely staying afloat,other countries such as China,India and the middle east counties I really don't know,i think it would depend on what agreements they share with japan? Anyhow these are some of my thoughts on the war im really looking forwards to reading the replies,and if you don't agree with something i wrote on top please correct me,Especially looking forward if anyone with actual military experience could offer there opinions and a basic timeline on how such war would proceed