In this Friday, Oct. 20, 2017, file photo, British Ambassador Kim Darroch hosts a National Economists Club event at the British Embassy in Washington. Britain's ambassador to the United States resigned Wednesday, July 10, 2019, just days after diplomatic cables criticizing President Donald Trump caused embarrassment to two countries that often celebrate having a "special relationship." Credit: Sait Serkan Gurbuz | AP

Boris Johnson isn’t yet in Downing Street, but he has already made his first major move as leader – and it was a demonstration of weakness. In one flick of the wrist, the front-runner in the Conservative Party leadership contest sacrificed a knight, exposed his queen, and signaled to his pawns that they are there to lay down their careers for his cause.

The knight is Kim Darroch, Britain’s ambassador in Washington. The leaking of his frank and critical views of the Trump administration – which the diplomat described as “uniquely dysfunctional” and “inept” – infuriated the U.S. president.

Nothing in Darroch’s private memos stands out from most mainstream commentary about President Donald Trump. Diplomats routinely send frank, if not always flattering, assessments back to their governments. That is part of their job.

But Trump wasn’t going to let these insults stand – especially when he has so much leverage with which to exact his revenge: Britain is relying heavily on the U.S. president to help fast-track a trade deal to cushion the economic blow of leaving the European Union. Trump understands this and plays it up at every opportunity.

The president took the extraordinary step of describing the ambassador of the U.S.’s closest ally as a “pompous fool” and a “very stupid guy” whom the U.S. would no longer deal with. That was tantamount to declaring Darroch persona non grata – a serious escalation of diplomatic confrontation normally reserved for instances of espionage, or to send a pointed message to a hostile country.

While Trump made Darroch’s position almost impossible, it was Johnson who sealed his fate. In a televised debate on Tuesday night, Foreign Secretary Jeremy Hunt – his rival in the leadership race – branded Trump’s attack on the ambassador as “disrespectful and wrong” and pledged to keep Darroch in place until his term expires at the year-end.

Johnson ran the other way. Instead of backing Britain’s man in Washington, he professed the importance of his relationship with the White House and that between Britain and America, as if it were in doubt. That lack of support from the likely next prime minister appears to have pushed Darroch into resigning.

Coming from a politician who has staked his entire career on a claim that British sovereignty is so sacrosanct that the country’s closest trading relationship should be sacrificed to bolster it, Johnson’s position looks remarkably supine. It is also incredibly short-sighted.

This isn’t a win for Brexiters, some of whom will have hoped that Darroch, a Europhile, could be replaced with an ambassador more sympathetic to their cause and the Trump administration. Johnson may have lost a knight for no advantage. Theresa May may well get to appoint Darroch’s replacement before she leaves Downing Street.

Meanwhile, Britain’s civil service will be on its guard; instead of working to advise their new leader and advance his agenda, the wide network of advisers on whom ministers rely for counsel – the pawns – may well now hold back.

The irony that Johnson has failed to grasp is that Britain’s EU membership, for all its drawbacks, amplified its soft power in the world. Now the U.K. is left a supplicant, appealing to the goodwill of a wholly transactional U.S. president. If there was any doubt as to what the “special relationship” means, Johnson just defined it for us.

Therese Raphael writes editorials on European politics and economics for Bloomberg Opinion. She was editorial page editor of the Wall Street Journal Europe. Dana Milbank is on vacation.