Nato summits are generally unremarkable affairs, but this week’s two-day gathering in Brussels will be an exception. European members of the transatlantic alliance are pondering their biggest conundrum since its creation almost 70 years ago: is the US a friend – or a foe?

Only 18 months ago, the question would have been dismissed as absurd. But the globally destructive impact of Donald Trump’s chaotic presidency has shattered conventional wisdom and left strategic and geopolitical certainties in ruins.

The problem is not only that Trump will again insist on the other 28 Nato members increasing their defence spending, on the specious grounds the US is being “ripped off”. It’s not merely that he has queried the founding treaty’s article 5 commitment to collective defence, or that he may close US military bases in Germany.

The more fundamental problem is that the US president is questioning the purpose of Nato, despite it having advanced US security and economic interests since 1949, undercut efforts to forge greater European unity that could have challenged US dominance, kept the Soviet Union/Russia at bay, and (mostly) maintained peace in Europe.

Bottom line: Trump simply doesn’t buy into, or understand, basic concepts such as collective security, burden-sharing, forward defence and the balance of power. He just doesn’t get it.

This myopic, isolationist view, consistent with his “America First” outlook, reflects Trump’s hostility to multilateralism in general. He scorns the UN, and has cut its US funding and boycotted its human rights council in Geneva. He repudiates World Trade Organisation rules, adopting unilateral, protectionist tariffs that spark trade wars and threaten European jobs.

Trump tore up the Paris global climate change treaty, pulled out of the UN-endorsed 2015 Iran nuclear deal so beloved of Europe, and recently urged France to follow Britain in abandoning the EU – an organisation he treats with contempt. He singlehandedly wrecked last month’s G7 summit of leading democracies in Canada, petulantly rejecting its conclusions and insulting his hosts.

More gallingly, Trump treats old friends worse than ostensible enemies, personalising political interactions and resorting to bullying, rudeness and open misogyny. Angela Merkel has been singled out for special abuse. At the G7 meeting, he tossed two Starburst sweets at the Germans chancellor and said: “Here, Angela, don’t say I never give you anything.”

President Trump with May on her visit to the US in 2017.

Photograph: Olivier Douliery/EPA

Ever since he grabbed Theresa May’s hand at their first White House meeting last year, Trump has treated the British prime minister with patronising disrespect. His crass interventions in British life, for example via tweets promoting the far-right group Britain First, and attacking London’s Muslim mayor, Sadiq Khan, were extraordinarily insulting. The Queen’s famous sang-froid may be tested to destruction when his visit to Britain begins on Thursday.

If Trump’s crude, nationalistic policies and uncouth persona were the only problems, the European allies might just cope. But in recent months, as he has jettisoned experienced advisers and his belief in his own infallibility has grown, Trump has moved from difficult partner to potential enemy.

The question grows ever more pressing: whose side is Trump’s America really on?

Trump’s sycophantic courting last year of the Saudi royals and China’s authoritarian president, Xi Jinping, were early indications of his preference for dictators over democrats. His recent summit with Kim Jong-un did nothing to curb North Korea’s nuclear arms buildup. But it did reveal Trump’s almost indecent love of raw power and ostentation.

This ugly trait will be on show again when he meets Vladimir Putin, Russia’s he-man president, in Helsinki on 16 July. Jon Huntsman, the US ambassador to Moscow, insists that Trump will focus on Russia’s “malign activity”, be it in Ukraine, in cyberspace, or in conducting chemical weapons attacks in Syria and Salisbury.

Trump has also promised to quiz Putin over covert Russian meddling that benefited his 2016 election campaign, activity confirmed last week by a US senate report. But will he really do so in the private, closed-doors summit he has demanded?

A more likely prospect is more crapulous fawning over an autocratic leader who exercises a mysterious hold over Trump and, most Nato members believe, threatens European security.

As with Kim in Singapore, Trump’s big day out with Putin in Helsinki will be noisily declared, by him, to be an outstanding success contributing to global harmony. If, as is suggested, the two men agree to extend the New Start nuclear arms treaty, that will be a rare plus.

But just as likely are unilateral, Nato-busting Trump moves to ease sanctions on Russia over Ukraine, a deal to keep Bashar al-Assad in power in Syria, the “normalisation” of Putin’s regime, and other concessions undermining the post-Salisbury western consensus.

In an augury of worse to come, Trump will also seek Putin’s support over Iran. US efforts to force regime change in Tehran are gathering pace, principally by halting Iranian oil sales and trying to starve out the mullahs. Even as European diplomats struggle to sustain open lines to Tehran, the US navy is gearing up for confrontation if Iran’s revolutionary guards retaliate, as threatened, by closing the Strait of Hormuz and blocking all Gulf oil exports.

Here, in a nutshell, is why Trump’s US increasingly poses a threat to Britain and Europe. In a reckless bid to impose his will on a sovereign people,he is risking a global energy crisis, a new war in the Middle East and the safety and prosperity of all America’s allies. With friends like him, who needs enemies?