The chief of Britain’s Foreign Office does nothing casually. He is a past master at saying nothing unintended in public. So when the Foreign Office permanent secretary, Simon McDonald, went before MPs this lunchtime after the devastating resignation of Britain’s Washington ambassador, Kim Darroch, his incisive directness was a revelation. It was the most eloquent evidence possible that the Darroch affair is not just a diplomatic storm but an existential challenge to Britain’s entire foreign policy.

What precedent was there, the foreign affairs committee chair Tom Tugendhat asked McDonald straight off, for the head of state of a friendly government to do what Donald Trump has done this week and make it impossible for Britain’s senior representative in that country to do his job? McDonald’s answer was monosyllabic, crisp and explosive. “None,” he said.

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Labour’s Chris Bryant followed up. Surely there were precedents from unfriendly countries such as Venezuela? “I know of none,” McDonald replied again. Not even hostile states have behaved like Trump, he insisted. Had there been some distant occasion when a British ambassador fell foul of the White House in such a way? There was, McDonald admitted, a “difficulty” in 1856, when President Franklin Pierce accused the British ambassador of recruiting Americans to fight in the Crimean war. The listeners in committee room 16 laughed, but McDonald did not.

And then came in many ways the most extraordinary remark of the lot. “Nothing like this has ever happened before,” McDonald told another MP. “There must be consequences. What they are in detail I can’t tell you this afternoon.”

I’ve known McDonald a long time. He was in the Washington embassy when the Guardian posted me there during Bill Clinton’s presidency. Later, he worked for foreign secretary Jack Straw back in London. More recently he has been Britain’s ambassador in Berlin and is an authoritative analyst of German politics and policy. For the past four years he has been the head of the diplomatic service in London.

McDonald is emphatically not an old-school British diplomat. He is a bit of an outsider, modern, approachable and fiercely loyal to his staff and colleagues. Getting a story out of McDonald is one of the hardest journalistic tasks I have encountered. I’ve never heard him talk in public the way he did yesterday. It tells you that this is really serious. He is not just angry: he’s looking into the abyss into which the British postwar worldview is sliding.

Darroch was brought down by three people. The first was the leaker of successive critical memos from the Washington embassy, who gave them to one of Britain’s most effective pro-Brexit journalists. The second was President Trump, who followed up on the leaks with a fusillade of personal insults and punitive actions of his own which brought Darroch to the brink. And the third was Boris Johnson, who knowingly refused to express confidence in Darroch during Tuesday’s ITV leadership hustings with Jeremy Hunt. It was Johnson’s action that led directly to Darroch’s inevitable resignation today.

Facebook Twitter Pinterest Boris Johnson, ‘who knowingly refused to express confidence in Darroch during Tuesday’s ITV leadership hustings’. Photograph: Han Yan/Xinhua/Barcroft Media

Each of these requires us to pause and digest the implications. First, consider the leaker. This must have been either a senior insider – a politician or diplomat – with access to highly classified material. Alternatively, it may have been an outside job, a hack or intercept, possibly by the agent of a state bent on mischief. Either way, these options are devastating for the practicality of diplomatic cables in the modern era. They are a reminder too of the extent to which Brexit subverts the workings of the British state. There’s been nothing like it for alternative loyalty since the Soviet spies of the cold-war era.

Second, consider Trump. He didn’t have to turbo-charge the leak. He will have had some around him who tried to stop him. But he did it once, and then he did it again. These were knowing acts, deliberate interventions, designed to weaken a country that thinks of itself, and is still often seen in Washington, as America’s special ally. They were a wilful assertion of power over Britain; an attempt to fire and hire Britain’s envoy. It was a crude act with implications for any country that seeks alliance with America or indeed with any other global power. Xi Jinping and Vladimir Putin will have been watching approvingly. For Britain, though, it is a stake through the heart of its entire postwar self-image.

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Finally, consider Johnson. Though he has mainly been in hiding while the Brexiters’ coup against Theresa May works itself through the Tory party to his benefit, Johnson’s ITV debate exposed once again a politician who can rival Trump for self-centred and ruthless opportunism. Tuesday reminded us that, in Johnson’s world, other people are always expendable. Ask the former London police chief Ian Blair, sacked by Johnson in one of the most disgraceful acts of his mayoralty. Ask Iran’s prisoner Nazanin Zaghari-Ratcliffe, innocent victim of Johnson’s lazy mind and incontinent tongue. Now ask Darroch, whom Johnson casually hung out to dry. Each of them simply got in the way. If Johnson becomes prime minister there will be more, much more, of this.

In the end, though, the Darroch affair is not just about personalities but more importantly about policy. For the past three years, British ministers from May down have tried to treat Trump as a temporary difficulty to work around with a pretence of friendship while deeper institutional links and common interests with America endure. Once Trump goes, they believe, the reset button can quickly be pushed to return to the status quo ante of supposedly special relations.

Yet it is increasingly hard to take this view seriously. Too much is changing on too many fronts. Trumpism may not be temporary. He may well be re-elected next year (as Darroch himself pointed out in a memo). The underpinnings of Trumpism, in the shape of populist nationalism and contempt for other countries, alliances and accords, go much deeper, both in the US and in Brexit Britain. If Johnson, or any other Brexiter leader, gets his way, Britain may once again embrace the US. But the America they embrace will not be the outward-looking republic of presidents from Eisenhower to Obama but an inward-looking exceptionalist country that seeks to disrupt everything about the international order. In such a world, Britain risks becoming the vassal of a capricious unilateralist state. Johnson or his successor would be Britain’s Carrie Lam to Washington’s Xi Jinping.

This is a new world. There are no precedents. Britain’s postwar belief that it is a unique bridge between Europe and the US is more rickety now than ever. The Brexiters are set on destroying one end of the bridge. Trump is equally bent on blowing up the other. As the bridge begins to collapse, so does the transatlantic foreign policy that Darroch and others have battled so hard to sustain. As Simon McDonald put it today to the foreign affairs committee, there will be consequences. There certainly will. And none will be good ones.

• Martin Kettle is a Guardian columnist