I just found out that there never was a peace treaty signed between the USSR/Russian Federation and Japan at the end of WWII. Technically speaking Japan and Russia are still at war :D
Aye, the same goes for North and South Korea. The issue is neither nation is actually willing to back up the fact that they are, at the most basic level, 'at war'. So I don't recommend using that as your baseline for a casus belli to go to war friendo.