When will Europe realise the American president is an antagonist , not an ally?

They were right to be worried. Within hours of arriving in Europe, Donald Trump was busied 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, maybe watershed moment for Europe, full of anxiety and loathing.

All of which invites the issues to: how far will Trump be allowed to go before leaders of the western republics eventually depict the line? How long until they recognise him as an antagonist , not an friend, 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 switching. 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 assaults, info warfare and election meddling- in the recent developments, 12 Russian intelligence officials were indicted for hacking emails during the 2016 election campaign. Then there are the chemical weapons cruelties 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, portending a whole new world of misery: Iran.

European countries tend to forget Washington's enduring post-1 979 vendetta with Iran. They also underestimate the depth of American ignorance. US diplomats have not worked in Tehran for almost 40 years. American legislators, 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 tyrannies of the Gulf, to unfairly portray Iran as pariah and international bogeyman.

For John Bolton, Trump's veteran national security consultant, 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 run. 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 consultant, 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 friends 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 bargain would spell survival for Bashar al-Assad, Syria's Russian-backed war criminal chairperson, while corroborating Moscow's Middle East ascendancy. It would mark a historic betrayal of pro-democracy, anti-Assad forces-out that helped battle Islamic State. It would be a maybe terminal tragedy for Nato, dividing 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.

‘ 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 outstrips 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 their own nationals uprising during last December's street protests, his evisceration of the 2015 nuclear deal, sweeping new sanctions, hypocritical protests about the rights and relentless attempts to demonise and isolate Iran's leaders. It all points one route. So, too, does the unilateral US global embargo on Iranian oil exports , beginning this autumn.

America's allies cannot duck this looming cyclone. White House pressure, direct political interference, the use of US fiscal and currency levers, bogus intelligence, public scaremongering, egregious neglect for democratic norms, the UN and international law and aggressive resentment to bridge-building and diplomacy- these are among the familiar techniques Trump is applying 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 simply taking place, is a second bankable breakthrough for Putin following his World Cup public-relations takeover. Ali Akbar Velayati, chief consultant to Iran's supreme leader, received a warm welcome in Moscow last week, where he was assured of Russia's continued relationship. 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 exportations. Is the prospect of war with Iran sufficient reason for the western republics to stand up to Trump at last? Is this emerging new” axis of evil”, connecting 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

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