The imperial city of Persepolis, ruined capital of Persia’s kings, rises from the desert north-east of Shiraz like a rebuke to invaders, ancient and modern. Its marble columns, many still standing, were erected about 500BC when inhabitants of the British Isles were capering around in animal skins and it was Greeks who posed the biggest military threat. Donald Trump’s America was a bad idea whose time had not yet come.

Britain’s recent history with Iran is, for the most part, shaming. Nineteenth-century imperialists and traders exploited and bullied, redrawing its borders with the Raj. British armies invaded and occupied and, in the 1920s, helped to elevate Reza Shah to the peacock throne. The ensuing era of autocratic rule sowed the seeds of the anti-western 1979 Islamic revolution. At Persepolis, graffiti left by Victorian army officers still defaces its pillars.

The US has since supplanted Britain as tormentor-in-chief, but Iranians have long memories. Many would agree with Mohammad Mosaddegh who, before the 1953 Anglo-American coup that ousted him as prime minister, told the US envoy Averell Harriman: “You do not know how crafty they [the British] are. You do not know how evil they are. You do not know how they sully everything they touch.”

Given this bitter legacy, and its other regional blunderings, it might be assumed Britain would fight shy of further intervention. Not a bit of it. This week the US slapped unprecedented sanctions on Iran’s senior leaders, suggesting diplomacy is at an end. Yet as Washington’s war drums beat ever louder, a familiar sucking noise can be heard above the din. It is the sound of Britain being inexorably drawn – again – into an avoidable, calamitous Middle East war.

What is truly astonishing is not that Trump and headbanger hawks such as John Bolton and Mike Pompeo are gunning for Tehran – they have been spoiling for a fight ever since they wrecked the 2015 international nuclear agreement. Nor should we be shocked at the daily escalations, provocations, insults and punishments inflicted on Iran. That’s par for the course when Washington turns bellicose.

Trump threatens ‘obliteration’ after Iran suggests he has a ‘mental disorder’ Read more

What should really chill the blood of British citizens is the way their own government – and the two men who want to be the next prime minister – are creating a situation, largely undiscussed and undebated, in which Britain will have no choice but to support a Trump attack on Iran, and worse, will have little hope of avoiding direct military involvement.

This senseless, heedless drift into another ill-conceived, unjustifiable and illegal conflict must surely stir alarming memories in the most complacent Tory heart. Do none of these people recall a similar made-in-America catastrophe in Iraq in 2003? Don’t the Chilcot report’s damning findings – that Tony Blair failed to explore all peaceful options and deliberately exaggerated the threat – ring urgent bells now?

The foreign secretary, Jeremy Hunt, supposedly the sensible Tory choice, is clinging uncomfortably to the White House bandwagon. Hunt knows full well American and Israeli claims that Iran is building a nuclear weapon are hyperbolic and disingenuous. But, he says, Britain under his leadership would “stand by the United States” and consider military intervention in Iran “on a case-by-case basis”.

Hunt has also made clear he accepts unproven US intelligence blaming Iran for recent attacks – indeed, he disowned a British general who questioned it. And he agrees with Trump that Iran’s ill-defined “destabilising activity” constitutes a casus belli.

Hunt may not want war. But a British military buildup is under way on his watch, including deployments of special forces, marines, Royal Navy ships out of Bahrain and, potentially, RAF F-35 fighter jets based in Cyprus. Trump has already demanded armed patrols to protect oil tankers in the Gulf. Just where does Hunt imagine all this will lead, if and when the US starts shooting?

Facebook Twitter Pinterest Donald Trump authorises ‘unprecedented sanctions on Iran’s senior leaders, suggesting diplomacy is at an end’. Photograph: Mandel Ngan/AFP/Getty Images

Boris Johnson has said less about Iran, hoping perhaps not to remind people of his mishandling, when foreign secretary, of the Nazanin Zaghari-Ratcliffe case. Yet his likely attitude to an outbreak of hostilities is no mystery. Johnson is an unabashed Trump-world groupie. He already has the president’s personal backing. And his Brexit strategy, such as it is, depends on swiftly cutting a comprehensive US trade deal.

If Trump turns to Britain when faced with Iranian retaliation – which would certainly have happened last week if planned US air strikes had gone ahead – arch-sycophant Johnson, like company-man Hunt, can be expected to fall tamely, even enthusiastically, into line. Will parliament or the public be consulted in a timely fashion? Not a hope!

It’s true the US takes ever less notice of what British leaders say – reflecting a wider problem of declining national influence. But Trump still needs moral ballast. The prospect of Britain sleepwalking into a new American war has been greatly increased by Tory-propagated delusions about the country’s global status and supposed ability to “punch above its weight”. Their bluff may soon be called. But it won’t be Tory blaggers who pay the price.

The Conservatives’ Brexit obsession has also distracted attention and blurred judgment. When the foreign office minister Andrew Murrison visited Tehran this week, his tough reception should not have come as a surprise. In their talks, Seyed Abbas Araghchi, deputy foreign minister, displayed an embarrassingly accurate grasp of where post-imperial, post-Europe Britain was going wrong.

“It appears that the British government’s entanglement with the predicament of Brexit has prevented it from having a correct understanding of the global realities,” Araghchi said acidly. Its collusion with “America’s bully-style measures” would only further harm the UK’s standing.

The Iranians are right. Across the Middle East, Britain is too often seen as in league with despots and murderers while its subservience to harmful American policies erodes its reputation. In Yemen, Britain is closely identified with a Trump-backed, Saudi-led war that has caused immeasurable suffering – and to what end, save further unlawful weapons sales?

When it comes to Palestine, Trump’s risibly biased “peace plan” is destroying the international consensus favouring a viable Palestinian state – a consensus consistently supported by Britain. In Syria, commendable British humanitarian aid efforts are sabotaged by US indifference, while in Egypt a dictator receives Trump’s blessing even as he scandalises universal human rights law.

If Trump’s hawks get their war, Britain risks being sucked in on the side of an aggressive superpower whose words and deeds are increasingly inimical to this country’s interests and values. There’s an old debt to be paid, and it’s high time Britain finally did the right thing by Iran. That requires unhesitating, active opposition to the threat the Trump regime poses to Iranians, the wider region – and to us.

• Simon Tisdall is a foreign affairs commentator