It depends. If North Korea were intending to bomb Japan, it'd be an act of war. I'm dubious they'd knowingly kick off a war by bombing Hokkaido though. So it'd

It depends. If North Korea were intending to bomb Japan, it'd be an act of war. I'm dubious they'd knowingly kick off a war by bombing Hokkaido though. So it'd more likely be an accident like the time the US bombed a Chinese embassy. https://en.wikipedia.org/wiki/United_States_bombing_of_the_Chinese_embassy_in_Belgrade https://en.wikipedia.org/wiki/United_States_bombing_of_the_C... Whether anyone accepted the accident angle would depend on how willing various major powers were wanting to get involved in the incident. But 9/11 would not be a good analogy, as it was not carried out by a nation. Which is why it took a month for the US to declare an illegal war on Afghanistan rather than immediately being able to declare a legal war in self defense.