So quickly has the unthinkable become unremarkable. US presidents never used to conspire to undermine European security. Nowadays it is normal. We learned last week that, when Emmanuel Macron was a guest in the White House in April, Donald Trump suggested France leave the European Union. And it hardly makes the top 20 Trumpian outrages of the year so far.

It isn’t news that Trump despises the EU. His primary grievance is economic: the US imports too many European goods (German cars, for instance). He believes that the strong sell to the weak, and thus a trade deficit is a symptom of national enfeeblement and a shame to be extirpated. So he launched a tariff war with Brussels. But that is a symptom of a more profound cognitive impairment. The president struggles with concepts of reciprocity and solidarity. His is a zero-sum universe in which benefits enjoyed by anyone else must have been deducted from his portion.

He also knows no history. He does not recognise the underlying ethos of the EU, conceived in the ashes of 20th-century apocalypse, binding formerly antagonistic states into mutual economic obligations. The very idea belongs to a dimension that Trump’s mind cannot visit. No wonder he likes Brexit.

It would be naive to imagine the present-day EU as a perfect realisation of its founding promise. And there is no available counterfactual to show how much poorer and less secure its members might be had their union never evolved. Still, its rise has generally tracked trends of unprecedented peace and prosperity, so it is rational to be afraid when the White House agitates for the whole thing to unravel.

Doubling pro-Europeans’ anxiety is the thought of Angela Merkel reaching her political twilight. The German chancellor is in her 13th year in office. She stands on the continental stage as an ambassador from the past and keeper of its lessons. Her childhood was spent in an authoritarian communist republic that was dissolved in 1990. Her career is a tribute to the merit in tearing down walls.

But her coalition government is fragile. The moderate, liberal consensus it upholds, and of which she has come to be an embodiment, looks haggard and defensive. The Europe that Merkel represents is besieged by populists and nationalists. The trend manifests itself in varied forms from country to country. The new maverick Italian strain is different to the entrenched Polish and Hungarian versions. But a common thread is venomous anti-immigration rhetoric in harmony with the Trump agenda. Richard Grenell, Washington’s ambassador to Berlin, recently gave an interview to Breitbart, the hard-right propaganda outlet, in which he described an ambition to “empower” disruptive movements spreading conservative dissent across the continent.

Consider what embattled European liberals make of Brexit in this context. It is admired by a US president who wishes misfortune on them; and that president is admired by Tory politicians who speak of Brussels as if it were a mortal enemy. From across the Channel, Trump and Brexit look like monstrous conjoined electoral twins, born a few months apart in 2016, both conceived in hostility to prevailing norms of global governance.

Theresa May understands this, and has tried to rebrand Brexit as something Europe-friendly. When speaking with an eye on her continental audience, she emphasises shared history and values. She talks of an enduring, close partnership. She believes it, too. The only significant intervention she made for the remain campaign in 2016 was a speech explaining how an alliance of western democracies amplified the UK’s power in the world. “The European Union does make us more secure, it does make us more prosperous and it does make us more influential beyond our shores,” May said.

One of the crippling delusions that fogged Brexiter judgment at the start of the article 50 process was a belief that individual national interests of the 27 other member states could be gamed to the UK’s advantage: that while the commission was formally in charge of the negotiations, there would come a point when old-fashioned bilateral bargaining could take over. Then the mythical “bespoke” deal – stitched from scraps of old treaty to fit around Britain’s economy – would be available. It hasn’t happened, and Trump is a large part of the reason. His marauding presence on the global stage enhances the value in European community and casts Brexit as its antithesis.

For every effort the prime minister makes to explain that Britain still wants to uphold the rules-based international order, there are a dozen times her cabinet, her party, and the whole frenzied Brexit-boosting carnival proves the opposite. There is Boris Johnson, fantasising aloud how much better Trump would be at handling the negotiations. There are reports that John Bolton, Trump’s national security adviser, held private talks with hardline pro-Brexit MPs behind May’s back. And these are the alt-right Tories who stalk the prime minister, daggers half-drawn, signalling that their revolution will be completed either by her or over her political corpse.

How is the EU supposed to accommodate a country whose leader claims to support its project but whose ruling party fizzes with excitement at the prospect of an epoch-shaking schism? How is Merkel or Macron to understand May’s ambition for a “deep and special partnership” when they can see the wreckers over her shoulder; when her friendly words are drowned out by drums that beat in perfect time with sworn enemies of Europe’s founding idea?

The prime minister has ducked many choices since the negotiations to leave the EU began, and avoided many hard questions. But they all flow from one strategic call; one irreducible Brexit dilemma. Our most valuable allies see their problem as the unravelling of European solidarity. Britain has to decide whether it is serious about being part of the solution.

• Rafael Behr is a Guardian columnist