Reports the Japan Times:
"But it is only Japan that expressed its wicked intention, letting loose a spate of balderdashes," the Foreign Ministry said, referring to comments that Tokyo won't accept a nuclear North Korea. "The Japanese authorities have thus clearly proved themselves that they are political imbeciles incapable of judging the trend of the situation and their deplorable position."
North Korea also said that Japan shouldn't participate because "it is no more than a state of the US and it is enough for Tokyo just to be informed of the results of the talks by Washington."
This bizarre tirade reveals something important about North Korea, namely that Pyongyang is just like the schoolyard bully who may not be the toughest kid around, but he's certainly the meanest -- and he is just as apt to use words as force to hurt others. This comment about Japan's being a "fifty-first state" goes straight to the jugular. It's the kind of concept that Japanese intellectuals are constantly debating as it encapsulates all the feelings of inferiority that come with Japan being the junior partner in the US-Japan alliance, "forced" to accommodate the presence of US forces on Japanese soil and all the spillover costs, material and psychological, that come with it.
As with any bully, Japan can either ignore the taunting or it can (theoretically) react violently; Japan will, of course, choose the former course of action. But the time may come when North Korea's even bigger neighbors may decide that they've had enough of its thuggery and act to end the DPRK's bullying once and for all.
This all goes to say that while international relations is a sophisticated discipline with its own jargon and cardinal concepts, it is important to remember that it is a social science -- concerned at heart with studying the behavior of humans in groups. Therefore its conclusions reflect not just state behavior but human behavior, particularly human behavior in settings that involve measurements of relative power and status and ego (whether individual or national).
It's not so much a matter of "everything I need to know I learned in kindergarten" but rather human beings in groups -- whether around a conference table in Beijing or in a children's playground -- act in similar (often similarly immature) ways.