Kim Darroch is but the latest victim of Donald Trump’s Death Star presidency, which collides at random with people and countries, sparking destruction and universal mayhem.

Many good men and women have suffered similar fates – shot down by blasts of laser-like presidential animosity. Like them, Britain’s departing ambassador to the US failed to treat Washington’s Dark Knight with fawning awe. Never mind. Johnson will set things right.

Except, perhaps not. The widespread assumption that a Boris Johnson premiership will instantly restore US-UK relations to brimming good health is facile. Trump’s inept, insecure, chaotic and dysfunctional behaviour, to borrow Darroch’s words, guarantees that Johnson, having betrayed a better man, will inevitably get zapped, too. He will richly deserve it, although it may be Britain that suffers.

That’s the problem with appeasement. Whatever you do, it’s never enough. Theresa May went out of her way to flatter and placate Trump from their very first White House meeting. He repaid her by mocking her over Brexit, talking up Nigel Farage and talking down Britain. Publicly calling the prime minister a fool was not the action of a friend.

Historical comparisons can be overdone at times of stress. Trump is not a born-again Hitler and his resurrected American brand of know-nothing rightwing populism is not a return to Nazi national socialism. He is probably not a fascist. But his ultra-nationalist chauvinism, ill-disguised support for white supremacists, racial and gender prejudices and vindictive temperament sometimes render him indistinguishable from a Cable Street blackshirt.

What Trump is, without any doubt, is an unsuitable partner for modern, multicultural, democratic Britain. His words and policies pose an unmistakable threat to the national interest, prosperity and values. For three years, ministers have clung to the hope that, somehow, he could be tamed – or at least, managed. But as Darroch noted in his leaked memos, Trump is not going to change. Last week’s events showed that this reality can no longer be ducked.

Security and intelligence-sharing are most often cited as the critical glue holding the “special relationship” together. In both areas, Britain and the US are at growing odds. Washington’s moves last week to create a naval taskforce to patrol Persian Gulf and Red Sea flashpoints sucks Britain more deeply into Trump’s manufactured confrontation with Iran. That country’s apparent attempt to seize a British tanker showed how easily hotheads on either side could spark a war.

The point is that Britain does not agree with Trump’s baiting of Iran, a policy Darroch termed “incoherent”. Ministers believe current tensions date from Trump’s decision last year to renege on the 2015 nuclear deal.

Britain shares concerns about Iran’s ballistic missiles and regional behaviour, but it also knows swingeing US sanctions are unjustified and provocative. It knows that the hawks advising Trump are trigger-happy.

Intelligence-sharing – another reason for putting up with Trump – is also in jeopardy due to the Huawei dispute.

Britain’s loyalty to the postwar US alliance has often worked to its disadvantage. It is hardly controversial to say that disastrous US or US-directed military interventions in Iraq, Afghanistan, Yemen and elsewhere, and systemic abuses arising from the post-9/11 “global war on terror”, have not served Britain’s interests or respected its values. Yet, under Trump, the UK’s self-defeating loyalty goes unreciprocated. See, for example, his coddling of Russia’s Vladimir Putin, the man who attacked Salisbury with a chemical weapon.

As its economic pre-eminence is challenged and its moral authority weakens, Trump’s America is increasingly relying on military might to maintain its global edge. Annual US defence spending is projected to rise to $750bn next year – upwards of 60% of the entire federal budget. That’s a rise of more than $100bn since Trump took office. The US is also, by some distance, the world’s leading arms exporter. Britain, by contrast, spends about $50bn annually on defence.

What are these vast amounts of arms and money for? The US already out-guns, out-nukes and out-missiles every nation on the planet. The obvious concern is that Trump’s America may increasingly seek to enforce its will, and impose its terms around the world, from behind the barrel of a gun.

Forget the UN, multilateral collaboration and the global rules-based order that Britain upholds and Trump abhors. The term “exceptional nation” may soon be replaced, in the eyes of the world, by “objectionable nation”.

On present trends, Britain risks becoming a mere satrap of this militarised empire, a vassal state of Trump’s America where Independence day is celebrated with tanks and bellicose bombast. This grim fate may be compounded by an unequal, post-Brexit “free trade” treaty that rides roughshod over environmental, regulatory and public health concerns.

Trump sucks up to dictators, sanctions the world, abuses migrants and reviles independent journalism. And why should the British people, attuned to the intensifying climate crisis, kowtow to a man who, denying climate change exists, shreds America’s environmental protections and boycotts the Paris climate accord?

Trump’s America is an ugly, dangerous creation from which old certainties recoil. The US alliance can no longer be relied upon. The Darroch affair is a timely warning to step back and take stock. And it’s no good saying Trump will soon be gone. The way the divided Democrats are behaving, he could still be calling the shots in 2025.