Gulf tankers are attacked and UK intelligence concurs with US officials that it was “almost certainly” Iran.

Why does Jeremy Corbyn spring to Tehran’s defence, claiming no “credible evidence about the tanker attacks” and that “the government’s rhetoric will only increase the threat of war”?

Was this an honest attempt to stop a rush to war by US “neoconservatives”? Or perhaps a cynical exploitation of a security crisis to appeal to anti-Trump sentiment and taint the Tories rather than Labour with the legacy of Iraq?

Whilst other European leaders have also withheld judgement on Iran, Mr Corbyn’s response cannot be separated from his foreign policy record, which suggests an even more troubling conclusion.

• JC foreign editor: The sanctions are biting, but unlikely to sway Iran. We're in for a long, hot summer in the Strait of Hormuz

Neither facts, reason nor national interest can release Mr Corbyn from his ideological straightjacket, according to which Washington poses the greatest threat to peace and any force confronting America is just.

“Whether it’s an attempt to remove Venezuela’s democratic government, or regime change in Iran, the USA is causing global instability in furtherance of its imperial interests.” That is not a parody but an actual tweet this week by Corbynite MP Chris Williamson.

Mr Corbyn may not typically speak quite that way — especially now that entering Number 10 is a real prospect — but this view is the thread that runs through Mr Corbyn’s approach to world affairs.

It is the opposite of Britain’s post-war foreign policy consensus which positions Britain as a trusted, though preferably not slavish, partner to America in advancing global stability, trade and liberal values.

The irony is that whilst distaste for Donald Trump may give Mr Corbyn’s anti-Americanism broader appeal, Mr Trump actually shares the Labour leader’s disdain for America’s role defending a liberal order. Had they spent time together during his recent visit, they may have discovered their weird commonality.

Each subscribes to a foreign policy impervious to rational calculation of interests, including opposition to America’s global role, sympathy for Vladimir Putin and disdain for one’s own intelligence services.

Mr Trump is also averse to military entanglements in the Middle East, as shown by his desire to pull troops from Syria.

Whilst National Security Advisor John Bolton may want regime change in Iran, Mr Trump seems mainly concerned with pulverising the Obama legacy. For all the maddening bluster, Mr Trump has repeatedly called not for war with the Islamic Republic but renegotiation.

In light of that, rather than bleating about Britain being “enmeshed in a huge potential war”, as Emily Thornberry did in an embarrassingly detached BBC radio interview, a serious opposition ought to challenge the government to work with partners on a credible strategy, pairing diplomatic outreach with a united response to Iranian security threats.

The goal — as set out in a recent Bicom paper — should be defusing the undoubtedly dangerous tension in the short run, whilst also seeking to plug some gaping holes in the nuclear agreement.

The government’s capacity to act is of course completely hampered by the Brexit crisis they have engineered, of which the Tory leadership election is only the present phase.

Tragically, if Mr Corbyn stumbles into Number 10 whilst Mr Trump is in office, Britain’s ability to play any meaningful role in the Middle East will be further diminished. The blow will be followed by a second whammy: an unprecedented nadir in UK-US relations.

Mr Corbyn and Ms Thornberry’s short-sighted insults — whilst no doubt personally and politically gratifying — will render them persona non grata in the White House.

As for Britain’s influence with players in the region, Labour long ago fell out with Gulf Arab leaders as well as Israel. Where would this leave a Corbyn government in a Middle East crisis? Isolated, ignored and ever more irrelevant.

Toby Greene is Senior Research Associate for Bicom and Research Fellow at the Leonard Davis Institute, Hebrew University