Though the president of the United States has fired off another volley of insults, his primary target – the British ambassador, Kim Darroch – has done nothing but his job. The national interest would hardly be served by Her Majesty’s chief representative in Washington sending back sanitised and euphemistic dispatches. Governments rely upon thorough, honest and frank information and advice from their diplomats. Sir Kim’s unflattering assessments of Donald Trump and his administration are embarrassing; but the problem was the leaking of the documents, not their drafting.

If the memos are unusually strong stuff, that is because the US administration is a wholly abnormal one. Indeed, the ambassador’s verdict of a dysfunctional, faction-riven and inept White House is not only blindingly obvious to any observer but looks decidedly diplomatic when set beside some of the accounts which have emerged from the leaky Trump administration itself. There are multiple reports of senior figures describing him as an idiot, a moron or unhinged.

Mr Trump’s response to these memos is yet more proof of his unsuitability for the office he holds. His attacks on the ambassador and on “foolish” Theresa May are further evidence of his bullying instincts. He embraces North Korean dictator Kim Jong-un and other autocrats, but treats the leaders of liberal democracies with condescension at best, and frequently contempt. He confuses aggression with strength and has only two modes: fulsome flattery, or battering an opponent into submission.

Like many bullies, he is also unable to take what he dishes out. His intemperate reaction rather reinforces Sir Kim’s observation that he “radiates insecurity”. Any politician must expect criticism. The commander in chief of the world’s superpower should surely be able to rise above a few remarks from a career diplomat. Instead, he has said he will not deal with a man he says he does not know but nonetheless pronounces pompous, very stupid, wacky and not well-liked.

Sir Kim may be able to overcome his personal disappointment at missing a state dinner with the emir of Qatar and even trade secretary Liam Fox’s meeting with Ivanka Trump, which he apparently sat out of his own volition. But this is serious stuff between allies: Mr Trump seems to want Sir Kim fired, as if Britain were a subsidiary of the Trump organisation.

Mrs May is right to back Sir Kim so clearly. Her successor should do so too. Jeremy Hunt already has. Sir Kim is due to step down early next year. A speedier departure, caving to White House tantrums, would only encourage Mr Trump’s bullying, and – one assumes – reward the leaker of these documents. The next prime minister might be tempted to announce the name of the next ambassador early, as an easier way of drawing a line under the affair. But that would make Sir Kim a lame duck. It is not for the president of the US to determine who has served the UK well and who should be its next ambassador to Washington – any more than it is appropriate for the president of the US to denigrate his British counterpart as he has done Mrs May.

Yet if this episode highlights many of the things that are wrong with the US administration, Britain does not come out of it looking entirely healthy and functional either. Though the source is unknown, the Brexit-friendly conduit and the way Nigel Farage leapt on the opportunity to put the boot into the civil service and demand a pro-Brexit political appointee have certainly raised suspicions. The fear is that Boris Johnson, the likely next prime minister, who boasts of good relations with the White House, might acquiesce. That too would be a mistake.

It is striking that people who insist that Britain must leave the European Union no matter what the cost, to “take back control”, see no problem with being browbeaten by the US. Perhaps the sole positive in this row is that it has exposed the cheapness of talk of the special relationship, and the attendant claims of a glorious US trade bonanza waiting for Britain once Brexit is complete. This attack on Britain’s leader and its ambassador might be considered a warning.