Iran’s seizure of a British oil tanker on Friday, following the UK’s seizure of an Iranian tanker a fortnight ago, marks a major escalation in the ongoing crisis in the Gulf. Under the 2015 deal between Iran and the world’s major powers, Tehran agreed to limit civilian uranium enrichment in return for the alleviation of sanctions and the opportunity to expand its international trade, primarily with Europe. Trump took the United States out of the deal in May 2018 and imposed a tough new set of sanctions on Iran.

The effect on the Islamic Republic has been severe, drastically shrinking its oil exports, plunging its economy into recession, and sending food and medicine prices soaring – even though Tehran’s compliance with the deal was repeatedly verified for a year after Washington pulled out.

Citing unspecified Iranian threats to US interests, in May this year Trump sent an extra aircraft carrier to the region, dispatched additional troops and deployed nuclear capable B52 bombers. In retaliation, Tehran appeared to launch a series of attacks on shipping in the Gulf, though it denied responsibility. Last month the Iranians shot down a US drone which they claimed had entered their airspace; Trump came close to launching missile strikes but backed down at the last moment.

The European signatories – France, Germany and the UK – have defied Trump and stuck with the nuclear deal, regarding it as a major diplomatic achievement with the potential to wind down regional tensions and open up the Iranian economy to European exporters and investors. Iran is now demanding that the Europeans honour their obligations under the deal (as it interprets them) by facilitating that trade and investment, circumventing Washington’s sanctions.

Iran’s recent minor breaches of the deal’s limits on uranium enrichment and stockpiling can be seen as relatively feeble cries for help. They do not raise any serious prospect of a nuclear threat, but show the deal could collapse if the Iranian economy does not gain some relief from Trump’s punishment. Europe could provide that relief, and has been developing a special purpose financial vehicle to make it possible, but progress has been painfully slow. Were the Europeans to come through, however, Iran ought to dial back its military and diplomatic countermeasures, and tensions in the Gulf would recede.

The weak link is Britain. In May, the leading British general in the anti-ISIS coalition in Iraq disputed White House claims of an increased Iranian threat. But the foreign secretary, Jeremy Hunt, sided with Washington against his own military officer on the ground. Unlike its European partners, Britain joined the US in swiftly blaming Iran for the subsequent attacks on Gulf shipping. And when Washington urged London to seize an Iranian oil tanker off the coast of Gibraltar this month, the British obliged.

London says that the tanker was taking Iranian oil to Syria in defiance of EU sanctions. Other European capitals were unimpressed by the claim, and Tehran flatly denied it, accusing the UK of piracy and threatening to impound a British tanker in response. It failed in its first attempt, prompting Hunt to announce that the UK would send an additional destroyer to bulk up the Royal Navy’s presence in the Gulf, but appears to have succeeded in its second attempt last Friday. By acceding to Washington’s demands to seize the Iranian tanker off Gibraltar, London has accelerated the drive to a war it does not want.

If Britain has been weak so far, it is set to get weaker still. The economic fallout from a no deal Brexit will leave Britain on its knees and – in its desperation for a trade deal with the US – even less willing or able to resist Washington’s demands.

Theresa May received little but contempt from Trump in return for her persistent fawning, but this hasn’t discouraged Boris Johnson from doubling down on the same failed strategy. When London’s ambassador to Washington had his disparaging evaluation of the president leaked to the press, Johnson – unlike May and Hunt – pointedly failed to back the diplomat, who therefore felt compelled to resign. If such servility continues, it will only make war with Iran more likely.