With Britain stuck up the Strait of Hormuz without a paddle, one of the most important decisions facing Boris Johnson’s embryonic premiership will be who to turn to so as to ensure we are not left all at sea.

Our new prime minister will likely consider it a rhetorical question. After all, someone who has spent a lifetime harbouring Churchillian fantasies knows there is only one truly dependable ally he can rely on when presented with the chance to render such reveries real.

One of the most intriguing reports about the Royal Navy’s seizure of the Grace 1 supertanker off the coast of Gibraltar came courtesy of Spain’s El Pais newspaper. It cited official sources who said the vessel had been under surveillance by US satellites since April. More importantly, it said US intelligence services provided the information that the supertanker was bound for the Syrian oil refinery of Banias.

Irans military fires a Sayad missile during the air defence drill (Picture: Iranian Army /AFPGetty Images)

There have been suggestions Britain was blindsided into doing the bidding of the US, but the more plausible alternative is that it knowingly entered the fray, with its fingers crossed that the US would come to its side.

Boris Johnson named new leader of the Conservative party

If such an approach seemed foolish when Iran seized the British-flagged Stena Impero tanker, the full extent of its folly was made explicit with Tehran’s claim – denied by the US – that it had arrested 17 alleged CIA recruits, and sentenced some of them of death.

Iran is reserving the worst of its provocation for the US, given its unilateral abandonment of the 2015 nuclear accord with the Islamic Republic, and the reimposition of severe economic sanctions.

As a result, Britain finds itself in an extraordinary bind: it is a minor player in a major conflict, and lacks both the capacity for a military response and the clout to influence a diplomatic one.

Even Mr Johnson, a man who yearns for the days when maps of the world were coloured with the empire’s pink, will surely realise we are in dangerous waters.

Brexit has already led to strained relations with our European allies, and their disquiet over the US approach to Iran’s nuclear programme, coupled with a new prime minister who lacks all credibility in the eyes of the EU, will fray them further.

All the talk of a European-led maritime mission to protect ships sailing through the strait, hurriedly announced in the dog days of an outgoing government, seems fanciful at best.

After all, it seems inconceivable that Mr Johnson will turn his back on Washington in favour of France and Germany – both co-signatories to the 2015 deal – over the increasingly vexed issue of Iran. The consequence is that Britain, under his watch, will go along with whatever approach the Trump administration decides is best.

At the moment, that takes the form of “maximum pressure” sanctions and embargoes, but given the occupant of the Oval Office is a capricious egotist whose only immutable foreign policy is the pursuit of American dominance, that could change quickly, especially if a belligerent Iran continues to goad him. It should not be forgotten that the Trump administration’s present Iran policy is borne from an uneasy compromise. There are powerful actors in the White House determined to sell Mr Trump – and the US public – the false choice of another war in the Middle East.

One official in particular, the hawkish national security adviser, John Bolton, is chomping at the bit for the US military to flex its muscles when it comes to Tehran. He has spent the best part of the past decade promoting pretexts for military strikes and, with them, regime change. Those in similar positions of authority in Mr Trump’s government have so far been able to curb Mr Bolton’s bloodlust, but given the rate of turnover among his senior advisers, it would be unwise to assume that uneasy status quo will hold indefinitely.

What hope is there that Britain can exert any kind of meaningful influence over this volatile situation? Our desperation for a favourable trade deal with the US has already brought about a grave imbalance in the so-called ‘special relationship’, and the leaks of Sir Kim Darroch’s diplomatic cables have strengthened the US hand further.

That shameful episode revealed just how far Mr Johnson will go to safeguard his own interests above those of his country, having repeatedly failed to defend one of its most senior diplomats.

As foreign secretary, he visited Washington last year in the hope of preventing the scrapping of the Iran nuclear deal, meeting with Mr Bolton, vice-president Mike Pence, and secretary of state, Mike Pompeo.

Quite what Mr Johnson’s hosts made of him is not clear, but given his pleas fell on deaf ears, the suspicion is he was little more than a clownish distraction.

The office of prime minister he will now occupy commands more respect, but its influence has withered dramatically in recent years. It will be diminished even more thanks to Mr Johnson’s character, temperament, and aptitude for strangling the truth.

In those respects, he will find common ground with Mr Trump, but even if he is sincere in his attempts to blunt the US president’s sharper edges in search of a bilateral solution, he has no leverage.

Bluster and bonhomie get you so far – they can even get you to Downing Street, it turns out – but Mr Johnson is about to find out the hard way the price of ambition.

A US war with Iran may be Boris Johnson’s first big test – Martyn McLaughlin