In such an exchange, each state needs three aspects of its words to do three things.
Their substance needs to convey exactly what that state means, their register needs to convey how relaxed or how angry it is, and the rank of their speaker needs to show how much authority they carry.
North Korea is careful on all three points. It expresses itself in uniquely colorful terms, but it usually uses standard phrases with quite precise meanings and deviates from these phrases only to signal a change in its thinking. The same goes for register.
Near-hysterical opprobrium of the US is routine and changes in the flamboyant language signal shifts in mood. Paradoxically this means that sometimes when North Korea is really angry, its language is almost sober. And it uses a carefully graded system of speakers to indicate how firm its statements are. At the bottom of the scale, remarks by unnamed state media commentators
are often just trial balloons. At the top, any remark attributed to Kim Jong Un is scripture, unchangeable and non-negotiable.
China is similarly systematic and careful.
While former US Presidents have, broadly, followed a similar system, which Pyongyang and Beijing could understand, President Trump's remarks -- and particularly his tweets
-- must have caused bewilderment.
Firstly, they come from the President himself, so in the traditional system carry the highest authority.
Secondly, his language is not only often stronger than that of former presidents, but also swings unpredictably. Although neither Beijing nor Pyongyang knew what "fire and fury" meant, it was clearly an entirely new formulation that sounded dangerous.
For a brief heady moment I thought this might be an act of perverse genius -- President Trump playing mad to give the Chinese the political cover they needed to get tough with North Korea. "Of course we are your friends, but we have to sanction you else who knows what the crazy American might do."
But no -- it turns out that the remark was simply reckless. Worse, President Trump then abruptly dropped this fiery language. Although welcome in itself, this will have caused further confusion. Was this an abrupt u-turn? What was the USA doing?
At first, Pyongyang tried to respond to President Trump's tweets with its own statements, but now seems to have stopped doing so. The regime seems to have concluded that presidential tweets do not mean anything -- a view that will have been reinforced by his literally incredible threat to halt US-China trade.
The effect is that the US has gagged itself. It is no longer able to communicate clear and credible messages to North Korea at the top level. At this time of tension, a miscalculation by North Korea would be unfortunate, and Pyongyang is more likely to miscalculate if it is unsure exactly what the US is saying to it.
It might get worse still. After his election, some thought that Trump might just be the man to cut the deal with Kim Jong Un. Dozens of wild tweets and remarks later, it is difficult still to believe this.
What if (big if) there were ever a summit between Trump and Kim at which the US President spoke as recklessly as he tweets? What might he give away to a well-briefed, affable and articulate Kim Jong Un, who would know just how to stroke President Trump's ravenous ego? Withdrawal of US troops from South Korea? Recognition of North Korea as a nuclear power?
It is quite possible that, having spent a fortune on nuclear weapons in pursuit of these objectives, Kim would be handed them for the price of a hamburger.