London: Iran's seizure and continued detention of a UK-flagged tanker deals Boris Johnson an immediate loyalty test: Britain's new prime minister may have to choose between Gulf escorts led by Europe or the United States.
Which way Johnson leans could set the tone for a complex agenda that includes withdrawing from the EU, striking a trade deal with the US, and maintaining or breaking European efforts to keep alive the deal curbing Iran's nuclear ambitions.
Some US commentators see this is a make-or-break moment for Europe's policy on Iran as a whole.
"Johnson could simply announce that the UK is joining America's maximum-pressure campaign and calls for a new (Iran nuclear) deal," the editorial board of The Wall Street Journal wrote.
"The rest of Europe would likely have no choice but to join its Anglophone partners -- and finally present a united front." The idea of a European-led mission in the Gulf is carried over from a meeting chaired by Johnson's predecessor Theresa May this week.
Britain has proposed that France, Germany and other European partners join together in a "naval protection mission" to ensure commercial ships can safely navigate in the Gulf.
But such an operation would expose Britain's continued reliance on EU allies at the very same time that Johnson is determined to yank his country out of the bloc on October 31, "no ifs or buts".
As an alternative, Johnson could instead sign Britain up to a US-led alliance outlined by Donald Trump's administration at NATO last month.
That decision could boost London's chances of reviving stalled efforts to strike a post-Brexit trade deal with Washington, but carries the risk that British warships could be caught up in more aggressive US rules of engagement.
Both Johnson and Trump played up their friendship during the British leadership race.
The US president cheered Johnson's election -- referring to him as "Britain Trump" -- and a source close to Johnson told The Daily Mail it was time to "reset" US-UK ties.
Yet that might doom British efforts to salvage the remnants of the 2015 deal with Iran that Trump pulled out of last year.
Tehran's ultra-conservative Resaalat newspaper published a cartoon Wednesday of Johnson as a British butler being patted on the head by Trump in the Oval Office.
"British Trump," the banner of the reformist Sazandegi said.
Johnson is yet to publicly comment on last Friday's capture by masked Iranian soldiers of the Stena Impero oil tanker in the Strait of Hormuz, at the entrance to the Gulf.
He will be expected to do so now.
His decision to push ahead with a European effort to secure the world's busiest oil shipping lane would still need to be coordinated with US forces in proximity to Iran.
Centre for European Reform foreign policy director Ian Bond, a retired diplomat who served in Washington and NATO, said Johnson might actually win Trump's favour by shepherding European navies to the Gulf.
"Based on the fact that Trump is always complaining about how little the Europeans do for their own defence, he ought actually to think that it was a good thing that the Europeans were taking care of this," Bond told AFP.
"But whether that is, in fact, how (Trump) will react I find it hard to say." Bond said Johnson's Brexit credentials might also be saved by the likely inclusion in this "coalition of the willing" of non-EU members such as Norway.
"This would be a practical implementation of what Theresa May was saying -- that we are leaving the EU, we are not leaving Europe," Bond said.
But Chatham House's Middle East researcher Sanam Vakil advised Britain's new leader to "avoid the temptation to align completely with Washington on Iran".
"Rather than conflating the ships and the nuclear crisis, a direct UK-Iran bilateral negotiation on the tankers could provide both sides with a face-saving outcome," Vakil wrote.
"The UK could position itself as a bridge between the EU and US, and in the process boost its post-Brexit relevance," he said.