With a transition of power underway in London, the nation is facing a gathering storm following Iran’s seizure of the UK-flagged oil tanker Stena Impero. This is a key challenge for incoming Prime Minister Boris Johnson, not just because he will have to navigate the spat with Tehran, but also complex, difficult diplomacy with the US and EU at a time when both alliances are under strain. The timing of this potential crisis is far from ideal for London. It coincides with Wednesday’s change of government — which will have a precarious hold on power thanks to its tiny majority in the House of Commons — and also comes at a time when Brexit deadlines are mounting, as the UK is scheduled to leave the bloc in under 100 days. On Monday, Theresa May’s outgoing administration decided that the best next step would be to “put together a European-led maritime protection mission to support safe passage of crew and cargo” in the Strait of Hormuz, through which “one-fifth of the world’s oil, a quarter of its liquefied natural gas and trade worth half a trillion dollars pass every year.” Within this diplomatic speak lies a potentially very important UK decision in favor of Europe’s continued support for the Iran nuclear deal. Inevitably, this has not proved popular with Atlanticist supporters of Johnson. Take the example of former defense secretary Michael Fallon, who on Monday challenged the government to declare that it would make sense for Washington to be included in the proposed force if it wished to join. May’s team admitted that the US had first requested the UK contribute to a US-led maritime protection force on June 24, leading to a formal request on June 30. It is therefore quite possible that, in the coming days, the new Johnson government will find ways for the European maritime force proposal to be sidelined, or potentially joined up in some way, with US assets and potentially those of other forces too, including NATO. What this highlights is that, underlying the Stena Impero’s seizure last week by the Islamic Revolutionary Guard Corps, there is a much bigger geostrategic issue in play. That is the fallout of Donald Trump’s decision last year that he would no longer recertify the Iranian nuclear deal — one of the biggest foreign policy choices of his term of office and which Johnson opposed when he was foreign secretary. Trump’s decision was immediately countered by French President Emmanuel Macron, German Chancellor Angela Merkel and May, who declared their nations would not just remain signatories to the nuclear agreement, but would work “collectively on a broader framework” with Tehran. For instance, Macron has indicated multiple times that Paris and other willing partners would work collectively on a new deal “covering nuclear activity, the post-2025 period, ballistic activity, and stability in the Middle East, notably Syria, Yemen, and Iraq.” Moving forward, while European allies would welcome the Trump team re-engaging with Tehran, this appears unlikely in the immediate future. Indeed, Trump asserted on Monday that Washington may be nearer war than negotiations with Tehran and that “we are ready for the absolute worst.” The ball, therefore, remains in the court of the continent’s leaders to try to preserve the 2015 nuclear deal, or remnants of it. However, one key uncertainty here is exactly what stance the unpredictable Johnson will take. The new prime minister is well aware that a key, growing challenge for Europe is not just Trump’s increased stridency against Iran, but also that President Hassan Rouhani has indicated his own weakening commitment to the 2015 agreement. Rouhani, for instance, has said Tehran will not reverse its decision to increase uranium enrichment beyond the limits set by the accord. The new prime minister is facing a very tricky situation on multiple domestic and foreign fronts. Andrew Hammond This shifting context makes Johnson’s decisions more complicated. Moreover, with a working majority of just two in the House of Commons, he must also navigate potentially difficult domestic politics at the same time as the Stena Impero issue, and also the fallout of the UK’s decision to apprehend the Iranian-flagged Grace 1 ship off Gibraltar for allegedly evading EU sanctions. While some leading opposition Labour Party figures have condemned Iran for its actions, including Shadow Chancellor John McDonnell, others warn of a slide to potential war. Take the example of Shadow Justice Minister Richard Burgon, who has told Johnson not to become Trump’s “sidekick” in a potential conflict that he argues could be worse than the 2003 Iraq War. The new prime minister is, therefore, facing a very tricky situation on multiple domestic and foreign fronts, meaning his political judgment faces a big early test. Not only must he try to de-escalate tensions with Tehran, while smoothing ties with Washington and key EU allies, he must do this when his grip on power is precarious, which — with Brexit challenges mounting — could yet trigger a general election.