Jeremy Hunt’s well-publicised dash to Brussels to save the world from war with Iran will be seen by his more cynical opponents as a last-minute bid to save his Conservative leadership hopes. Before attending an EU foreign ministers’ meeting on Monday, Hunt declared portentously that the prospect of Middle Eastern states acquiring nuclear weapons posed “an existential threat to mankind”. That’s scary stuff, certainly deserving of a seasoned statesman’s full attention.

Showboating aside, what the foreign secretary’s Brussels mission really demonstrates is that unilateral British efforts to defuse the Iran crisis have failed miserably. In particular, this failure has cruelly exposed the hollowness of Britain’s claims to wield special influence in Washington’s corridors of power. Donald Trump could not give a fig what Britain thinks. All efforts to change his mind (including two Boris Johnson trips to Washington as foreign secretary) have flopped.

If Trump starts a shooting war, as he nearly did last month, Britain will be instantly stuck in the middle of the fight

If the Kim Darroch affair has any lasting significance, it is not the angry spat that followed the leaking of his private criticisms of Trump. It is the banality of the ambassador’s opinions. In characterising the administration as inept and dysfunctional, Darroch merely echoed the settled view of most independent Washington observers. He offered few insights, perhaps because he lacked any great degree of access. US policy on Iran, he wrote, was “incoherent”. No big surprise there.

British attempts to influence Iran have been similarly unsuccessful. Hunt visited last November in a bid to bolster the 2015 nuclear deal after Trump reneged on it. Such contacts became possible when full diplomatic relations were restored in 2016. In contrast, no US diplomat has set foot in Tehran since 1981. Official Washington’s ignorance of Iran is profound – and dangerous. Part of the British pitch to Trump’s people is that they are on the ground and know Iran’s thinking.

This claim, too, looks threadbare these days. When Andrew Murrison, a junior Foreign Office minister, travelled to Iran last month, he received short shrift from his hosts. They repeated their view that Britain was a mere tool in Trump’s clumsy hands – and would pay a heavy price for its self-abasement. Britain has also failed in repeated efforts to secure the release of Nazanin Zaghari-Ratcliffe, an entirely separate but nevertheless symbolic issue.

Against this backdrop of diplomatic limbo, Hunt’s high-profile resort to multilateral diplomacy in Brussels underscores the continuing, vital importance of the European alliance. It is also an admission that Britain is largely powerless on its own to influence events that threaten its security and prosperity. In this sense, the Iran crisis marks an important, even watershed moment in the Brexit saga. Hunt, like Boris Johnson, is campaigning to be prime minister on a firm pledge to drag Britain out of the EU. Yet where does he turn at a moment of extreme international stress? To indispensable Europe.

The Iran crisis has become a cautionary case study in the cold realities of Britain’s prospectively diminished, post-EU place and standing in the world. What a united European approach means in practice was exemplified by the joint statement issued on Sunday evening by Britain, Germany and France calling on the US and Iran to think again and start talking. This opening represents the best and perhaps last hope of avoiding a descent into war.

The hazards implicit in Britain acting without Europe are also dangerously clear. Having seized an Iranian oil tanker off Gibraltar, allegedly on US orders, and begun a naval buildup in the Gulf, again at US bidding, Britain has placed itself, its armed forces and energy supply-line squarely in the gunsights of Iran’s Revolutionary Guards. If Trump starts a shooting war, as he nearly did last month, Britain, without a vote or a voice, will be instantly stuck in the middle of the fight.

It is by no mean certain that European states, acting in concert, can persuade the two main antagonists to back down. The joint statement called on Iran and the US “to act responsibly and to look for ways to stop the escalation of tension and resume dialogue”. That was an unmistakable rebuke to Trump. The EU continues to pursue barter arrangements aimed at circumventing US sanctions, while warning Iran to honour its commitments. Emmanuel Macron, France’s president, has meanwhile proposed a time-out, meaning a freeze of the status quo.

What a truly extraordinary mess the Tories have got the country into. It’s evident that on Iran, as on most other major international issues, Britain naturally inclines to the European position. It’s clear the government agrees with Darroch that Trump’s trashing of the nuclear deal was a wanton piece of “diplomatic vandalism”. Hunt’s warning about the risks of having “different parties armed with nuclear weapons” in the Middle East was a reference to Saudi Arabia, whose nuclear ambitions are being actively encouraged by Trump, as well as to Israel and Iran. Hunt’s proffered compromise on the Gibraltar tanker is another sign of British dissent from the US line.

Iran refuses to end breach of nuclear deal until it gets 'full rights' Read more

Yet such minor, cautious gestures will not by themselves alter the current dynamic, which points towards military conflict, by accident or design. Not can they obscure the gross contradiction at the heart of the Tory Brexiters’ position – the rank hypocrisy of trying, even at last knockings, to have it both ways. By continuing to insist that Britain will leave the EU, Hunt (like Johnson) irresponsibly undermines Europe’s collective clout at what he acknowledges is a critical moment – and thus significantly increases the chance of war.

The ability of Europe to act effectively as a necessary counterweight to irresponsible, authoritarian and power-hungry rightwing regimes in the US, China and Russia hangs in the balance. What happens now may permanently affect the course and outcome of future crises. Yet what’s more important? One man’s leadership chances? The crazed Tory obsession with Brexit? Or acting honestly together with European friends and neighbours, for reasons of principle and common sense, to face down the global warmongers and keep the peace?

• Simon Tisdall is a foreign affairs commentator