They were right to be worried. Within hours of arriving in Europe, Donald Trump was busy insulting America’s closest friends and threatening to dismember Nato. He publicly humiliated Theresa May and did his importunate best to force regime change in Westminster, before halfheartedly apologising. Now he takes his ugly brand of rogue-male politics to Helsinki for a meeting with his best buddy, prominent campaign supporter and fellow narcissist, Russia’s Vladimir Putin. This is an ominous, possibly watershed moment for Europe, full of fear and loathing.

All of which invites the question: how far will Trump be allowed to go before leaders of the western democracies finally draw the line? How long until they recognise him as an antagonist, not an ally, contemptuous of their countries’ values and interests – and act accordingly? Germany’s Angela Merkel tried firmness. May tried flattery. The EU has tried fulmination and retaliatory trade tariffs. Others, wishfully, dismiss Trumpism as an aberration, not a strategic shift. But nothing stops him as he rampages on, malignly flattening all in his path.

It is plain what a US president should be talking to Putin about: Russia’s illegal occupation of Crimea, its cyber attacks, information warfare and election meddling – in the latest development, 12 Russian intelligence officials were indicted for hacking emails during the 2016 election campaign. Then there are the chemical weapons atrocities in Syria and Salisbury, Russia’s treaty-busting nuclear build-up and its sanctions-busting in North Korea. But topping Trump’s personal agenda, it seems, is something entirely different, presaging a whole new world of misery: Iran.

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European countries tend to forget Washington’s enduring post-1979 vendetta with Iran. They also underestimate the depth of American ignorance. US diplomats have not worked in Tehran for almost 40 years. American politicians, businesses and media have scant knowledge of the country. It has been far too easy, in such a vacuum, for its enemies, notably the paranoid Sunni Arab dictatorships of the Gulf, to unfairly portray Iran as pariah and international bogeyman.

For John Bolton, Trump’s veteran national security adviser, and others of his ilk, Iran is unfinished business, a part of George W Bush’s infamous “axis of evil”. Saddam Hussein’s Iraq was bloodily subjugated; Kim Jong-un’s North Korea is being brought to heel, too, or so they believe. That’s two down and one to go. Eager for a clean sweep, Bush’s more incompetent heir is taking his accelerating campaign against Iran to Putin’s door. There is very little Trump would not do to win Moscow’s support for his coming offensive.

As Susan Rice, Barack Obama’s national security adviser, has noted, Trump’s one-on-one Helsinki session with the vastly more experienced, cannier Putin – with no officials or note-takers present – is a “recipe for disaster”. Given his bottomless need for validation, the rumoured “grand bargain” on Iran, however costly, could be too much for Trump’s ego to resist; witness his fake news “triumph” at the North Korea summit in Singapore.

Western allies fear that Trump, egged on by the Saudis and Israel’s prime minister, Benjamin Netanyahu, who conferred with Putin in Moscow last week, may privately offer de facto US recognition of Crimea’s annexation and an easing of Ukraine-related sanctions. In return, he would seek Putin’s agreement to push Iran out of Syria, thereby safeguarding Israel’s border and weakening Tehran’s influence in Lebanon and the wider region.

Any such deal would spell survival for Bashar al-Assad, Syria’s Russian-backed war criminal president, while confirming Moscow’s Middle East ascendancy. It would mark a historic betrayal of pro-democracy, anti-Assad forces that helped battle Islamic State. It would be a possibly terminal calamity for Nato, splitting the alliance and potentially destroying its credibility in eastern Europe. And it would wreck Britain’s efforts to punish Putin after the Salisbury nerve gas attacks.

Facebook Twitter Pinterest ‘Israel’s prime minister Benjamin Netanyahu conferred with Putin in Moscow last week.’ Photograph: Jack Guez/AFP/Getty Images

But Trump cares nothing for such fallout, for his ultimate, undisguised objective far exceeds the mere stifling of Iranian ambitions. It is nothing less than all-out regime change. Anybody who recalls the build-up to the Iraq war can read the signs: Trump’s incitement of a national uprising during last December’s street protests, his evisceration of the 2015 nuclear deal, sweeping new sanctions, hypocritical protests about human rights and relentless attempts to demonise and isolate Iran’s leaders. It all points one way. So, too, does the unilateral US global embargo on Iranian oil exports, beginning this autumn.

America’s allies cannot duck this looming storm. White House pressure, direct political interference, the use of US financial and currency levers, bogus intelligence, public scaremongering, egregious disregard for democratic norms, the UN and international law and aggressive hostility to bridge-building and diplomacy – these are among the familiar methods Trump is employing to bully individual European governments into supporting his anti-Iran attrition and penalise them if they refuse. Yet in Putin’s case, kowtowing cravenly before imagined strength, he offers carrots, not sticks, plus large dollops of crapulous fawning.

It is by no means certain that Russia will play along. The Helsinki summit, by merely taking place, is a second bankable breakthrough for Putin following his World Cup public-relations coup. Ali Akbar Velayati, chief adviser to Iran’s supreme leader, received a warm welcome in Moscow last week, where he was assured of Russia’s continued friendship. Nor is it clear Putin has the power to eject Iran from Syria even if he wished.

But rational considerations will not deter an out-of-control Trump and his willing coalition of rightwing ideologues, unelected dictators and Israeli hawks from picking a fight if, for example, Tehran blockades the Strait of Hormuz and halts all Gulf oil exports. Is the prospect of war with Iran sufficient reason for the western democracies to stand up to Trump at last? Is this emerging new “axis of evil”, linking Trump, Putin and Assad, the ultimate abomination that finally forces our leaders to say enough is enough? War with Iran could make Iraq look like a walk in the park. Yet who, if not us, will stop him?

• Simon Tisdall is an Observer columnist