The biggest faction of foreigners in Japan are Koreans —there are two national groups representing them, and the bigger, more influential one believed it or not is the one loyal to the NORTH:
Every year they get milked to repatriate money (from grey-world gambling ops and grilled BBQ restaurants, among others) to the North. Used to be much more complex, but the authorities sort of cracked down on them about 9 years ago. Those shenanigans figured more in the DRPK’s revenue stream than most realize, though I don’t know how big it is now.
My guess is these were Nork agents —they’re trying to give Kim Jong Un a symbolic victory so he can save face, claim some type of victory and back away from a major confrontation he’s getting himself caught up in vis-a-vis South Korea.
"If we made 100 promises of 'Seas of Fire' we can't very well go and do NOTHING after our stupid box land-mines and couple of shells, the other day..."
So we'll be hearing of 10,000 Norky TV broadcasts screaming about how they nuked US forces in Japan, and our guys are crawling the streets, disemboweled and ashamed of challenging the Dear Leader, blah, blah, blah.
And there are still a bunch of subs out there, so maybe they'll try to do a torpedo job (again).
And lets not forget that the Norks have been kidnapping kids out of Japan to raise up as Nork agents to be reinserted back into Japan for decades now.