A SUPERTANKER LOADED with oil headed east from the Strait of Gibraltar into the Mediterranean, bound for Greece, just before midnight on August 18th. Nothing unusual in that—120,000 vessels navigate the strait each year, carrying a third of the world’s oil and gas. But the Iranian-flagged Adrian Darya 1 was no ordinary tanker.

Days earlier she had been renamed and reflagged, with fresh paint covering her old identity, Grace 1. That accompanied her release from over a month of detention in Gibraltar, a British overseas territory. The episode not only marked the latest chapter in a bitter struggle between Iran and America. It also highlights Britain’s strained effort to balance its Iran policy between American belligerence and European emollience.

The backdrop is President Donald Trump’s withdrawal last year from a multinational nuclear agreement that his predecessor, Barack Obama, reached with Iran in 2015. This deal had limited Iran’s nuclear programme in exchange for relaxing sanctions. But American sanctions were tightened sharply in April under Mr Trump’s policy of “maximum pressure”. Iran has hit back, apparently attacking ships in the Gulf of Oman in May and shooting down an American spy drone on June 20th. The latter caused Mr Trump to order—and then cancel—air strikes on Iran.

That was the febrile atmosphere on July 4th, when British marines abseiled onto the deck of the Grace 1 in Gibraltarian waters. They did so on the basis of American intelligence suggesting the tanker’s oil was bound for Syria, whose main refinery is under EU sanctions. On July 19th Iran procured a bargaining chip by grabbing a British tanker, the Stena Impero, in the Strait of Hormuz. That may have strengthened Iran’s hand. But it was not until it promised that the Grace 1 would not be sent to Syria, or any other forbidden destination, that Gibraltar agreed to release the vessel.

That was a relief to Britain. The threat of further ship-grabs would diminish, and Iran would probably release Britain’s tanker in turn. But with the Adrian Darya 1’s paint barely dry, a court in Washington, DC , issued a warrant for its re-seizure, on the basis that the ship was secretly controlled by Iran’s Islamic Revolutionary Guard Corps, which America considers a terrorist organisation. That cut no ice with Gibraltar. It pointed out that the EU ’s sanctions laws differ from America’s, and that Europe—like the rest of the world—does not deem the Revolutionary Guards terrorists.

The diplomatic tussle has put Britain in a tight spot. Like its European allies, it wants Iran to stick to the nuclear agreement. In January Britain spearheaded, with France and Germany, the creation of Instex, a barter mechanism to enable certain Europe-Iran transactions, particularly for humanitarian supplies. British diplomats are eager to soothe Iran with trade. The seizure of the Grace 1 threw a spanner in the works, not least because Britain was prodded into action by American intelligence.

Boris Johnson, who succeeded Theresa May as prime minister midway through the tanker crisis, has adopted a confrontational position towards the EU regarding Brexit. He has also courted Mr Trump, who is holding out the prospect of a post-Brexit trade deal to help cushion the British economy.

That leaves Britain’s Iran policy in a parlous position. Should Mr Johnson keep siding with the Europeans, he may anger America, risking the promise of a trade deal. But if he backs Mr Trump and heaps pressure on Iran, he could cause the collapse of the nuclear deal and a spat with Europe. “You can’t maintain that post-Brexit you’ll be a loyal ally of European nations in matters of security and ditch 20 years of Iran policy by moving over to the American side,” says Sir Richard Dalton, Britain’s ambassador to Iran in 2003-06.

So far Mr Johnson has sought a balance. Whereas Mrs May tried to organise a European security mission for the Gulf, Mr Johnson signed up to an American-led scheme. John Bolton, Mr Trump’s national security adviser, noted with pleasure that the choice “reflects a change from the prior government”. But the Adrian Darya 1’s release shows that even Mr Johnson is unwilling to break with Europe and join Mr Trump in throttling Iran. At least not yet. ■