AFTER MORE than a week of warnings, Iran has made good on its threat to seize a British oil tanker. The Stena Impero, a 30,000-ton vessel, was travelling west through the Strait of Hormuz on the afternoon of July 19th, bound for Saudi Arabia. Then the ship’s owner says it was “approached by unidentified small crafts and a helicopter.” It soon lost contact. That evening, tracking data showed the tanker and its 23 crew members in Iranian waters off Qeshm island, where the Islamic Revolutionary Guard Corps (IRGC) maintains a base. The IRGC says it was seized for “failing to respect international maritime rules”. A second vessel, the Mesdar, flagged in Liberia but operated by a British firm, was also reportedly diverted to Iran. Iranian news agencies said it had later been released.

Iran threatened to do exactly this after Britain seized an Iranian tanker off Gibraltar on July 4th. Britain believes the vessel was carrying Iranian oil to Syria, which is under European sanctions. Iran denies this and accuses Britain of breaking international law. Gibraltar’s supreme court extended the ship’s detention on July 19th, allowing authorities to hold it for another month.

These vessels are now pawns in a worsening crisis caused by America’s decision to walk away from a 2015 nuclear deal with Iran and impose crushing economic sanctions. Though Iran remained in compliance for a year, in recent weeks it has breached provisions on the quantity and purity of its enriched uranium. It is eager to show it can impose costs on America and its allies. Six tankers were sabotaged in the Persian Gulf in May and June. On July 10th Iranian forces attempted to halt a British tanker, reportedly retreating only when a British warship trained its guns on the Iranian boats. But Britain admitted it did not have enough warships to protect all of its commercial vessels.

The skies over the Gulf are growing tense as well. On July 18th the USS Boxer, an American warship loaded with marines, zapped an Iranian drone that had approached within 1,000 yards and refused to “stand down”, according to President Donald Trump. The drone was taken out not by bullets or missiles, but by electronic warfare used to jam its signals. The Boxer and five other ships had been passing through the strait as a show of force, and Iranian helicopters and ships had been making close passes, according to the Wall Street Journal. Iran claims, however, that all its drones are accounted for. One month earlier, Iran shot down an American drone.

All of these earlier incidents were, in a sense, small enough to overlook. Investigators have yet to assign blame for the sabotage operations (and Iran denies it was involved). Mr Trump approved airstrikes after the American drone was shot down, only to cancel them when he decided the response was disproportionate. Seizing tankers on the high seas, however, will be impossible for Iran to disown, and for America and its allies to ignore. Freedom of navigation is a core principle of international law. It will undermine Iran’s strategy of trying to drive a wedge between America and European powers, which remain parties to the nuclear deal and want to preserve it.

Ironically, hours before the tankers were seized, Iran’s foreign minister made a diplomatic proposal to end the crisis. A modest one, to be sure: Muhammad Javad Zarif offered to give international inspectors greater access to Iran’s nuclear facilities if America would lift sanctions. That was meant to happen anyway in 2023, as part of the nuclear deal. Mr Zarif’s offer would have simply accelerated the timeline. But it was the first concrete proposal from an Iranian government that, less than a month ago, said Mr Trump’s sanctions meant the “permanent closure” of diplomacy.

Making cosmetic tweaks to an old pact and slapping Mr Trump’s name on it is a tried-and-tested trick. In November America’s trade deal with Canada and Mexico was changed slightly, renamed USMCA and then embraced by Mr Trump. “Minimal changes could be enough to call it the Trump Agreement,” says Ernest Moniz, who played a key role in negotiating the original deal as secretary of energy in the Obama administration. But there is a small problem with attempting this solution for Iran, points out Nick Miller, a professor at Dartmouth: “to execute it, Trump would need to fire or ignore most of his national security team.”

Mr Trump’s advisors have been busy trying to box in the president. In May last year Mike Pompeo, the secretary of state, insisted that Iran cease all enrichment as one of a dozen conditions to be laid down for a new deal. On July 18th John Bolton, Mr Trump’s national security advisor, tweeted that “one of the worst mistakes of the Iran deal, now on full display, was allowing Iran to maintain enrichment capabilities.” Such drastic new prohibitions would go well beyond the face-saving modifications that Iran is likely to accept. More skirmishing probably lies ahead.

Editor's note: This is an updated version of a piece published earlier on July 19th 2019.