LONDON — For Boris Johnson, the G7 will be a tale of two Donalds.

The new U.K. prime minister flies into Biarritz Saturday lunchtime, determined to show the world Britain has its mojo back. The centerpiece of his weekend is a meeting first thing Sunday with Donald Trump. Hours later, Johnson will come face to face with Donald Tusk.

"My message to G7 leaders this week is this: The Britain I lead will be an international, outward-looking, self-confident nation," Johnson said in his pre-summit statement. Those who think Brexit — even the no-deal Brexit he is prepared to enact in just 10 weeks' time — means the U.K. retreating from the world are "gravely mistaken," he added.

If the Trump meeting is likely to be hailed by Johnson as an exemplar of the U.K.'s new place in the world, the Tusk one will be a reminder of the fractious Brexit process he needs to navigate before he can pursue that agenda with gusto.

In truth, neither head to head will be easy. While in style Johnson might appear to share the straight-talking rambunctiousness of his North American ally, in substance many of the prime minister's policy positions remain closely aligned with those of his continental counterparts. Play too nicely with one and he risks antagonizing the other — or worse still, being stranded, lonely in the middle.

Trump might talk up his good relations with Johnson, choose the prime minister as his first bilateral of the summit and discuss the agenda with him by phone beforehand, and Johnson may enjoy sharing the limelight with a U.S. president, but the two countries are far apart on key foreign policy questions, particularly on Iran and climate change. Regarding a new U.S.-U.K. trade deal, despite public confidence, officials in London are under no illusions about the serious hurdles that need to be overcome.

As for Tusk-Johnson, the European Council president said in February said there is a "special place in hell" for those who championed Brexit without a plan to carry it out. Most thought he was referring to Johnson, the champion of the 2016 Vote Leave campaign, who was then a backbench MP opposing Theresa May's Brexit deal.

As fellow leaders, their diplomatic relations haven't been much more cordial. This week, Tusk responded to a letter from Johnson requesting that a key pillar of May's deal — the Northern Ireland backstop — be removed, by effectively accusing the U.K. prime minister of wanting to reimpose a hard border on the island of Ireland.

And despite some warm words and mixed messages from Emmanuel Macron and Angela Merkel this week, a new deal before October 31 that could avert an economically disruptive, crashout Brexit still looks unlikely, and Johnson knows it.

"I want to caution everybody, OK? Because this is not going to be a cinch, this is not going to be easy. We will have to work very hard to get this thing done," he told broadcasters on Friday. No wonder then that the other message Johnson intends to convey to Tusk on Sunday, according to officials, is that the U.K. really means it (this time) when it says it will leave with no deal.

Looking both ways

The paradox of Boris Johnson's G7 agenda is that while the Trump meeting is at its heart, his other key messages could hardly sound more different to those of the U.S. president.

A convinced advocate of global action to combat climate change and protect the environment, the U.K. prime minister joined French President Emmanuel Macron in advance of the summit in highlighting the seriousness of the "heartbreaking" Amazon rainforest fire.

On Russia, he opposes Trump's wish to invite President Vladimir Putin back into the G7 fold, and on perhaps the most pressing foreign policy issue on the summit agenda — Iran — he stands with France and Germany.

U.K. officials indicated in advance of the summit that there is no change in the U.K.'s support for the nuclear deal, and that London, despite the change of leadership, is still not supportive of the U.S. "maximum pressure" on Iran strategy.

New Foreign Secretary Dominic Raab is, one official said, more inclined to a hard-line stance on Iran than his predecessor, Jeremy Hunt (Johnson's vanquished leadership rival), but the overall direction of U.K. foreign policy — on this and much else — has not radically changed.

This much was clear from Johnson's pre-summit statement.

"We face unprecedented global challenges at the very time when public trust in the institutions designed to address them risks being undermined," he said. “International tensions and new trade barriers are threatening global growth. Violence and conflict are trapping countries in poverty, depriving children, and particularly girls, of the universal right to education. Climate change is accelerating the devastating and unprecedented loss of habitats and species."

It's not a pitch tailor-made to appeal to Donald Trump.

Nor will the two leaders necessarily find an easy path to a new trade deal between their countries. In private, U.K. officials recognize the difficulties involved in securing a post-Brexit trade deal with the U.S. — namely ensuring protection for U.K. farmers and the NHS, while persuading Washington to open up the U.S. market to the U.K.'s services firms.

Talk of a series of mini-deals, promoted by Trump's National Security Adviser John Bolton on a recent visit to London, is not considered viable in London, and Downing Street is downplaying any suggestion of a timetable for negotiations emerging from the Trump-Johnson meeting.

“The prime minister and president have both repeatedly expressed their commitment to delivering an ambitious U.K.-U.S. free-trade agreement and to starting negotiations as soon as possible," a Downing Street spokesperson said. “Of course we want to move quickly, but we want to get the right deal that works for both sides.”

Johnson's real fight back home

And then, of course, there's Brexit.

Angela Merkel's suggestion on Wednesday of a 30-day timetable to find a solution briefly raised hopes in the U.K. that the EU might climb down from its refusal to countenance a reopening of the legally binding Withdrawal Agreement struck with May.

However, EU officials have been clear that there has been no change of position and that it is still up to the U.K. to come up with what Tusk on Monday called "realistic alternatives" to the Northern Ireland backstop plan.

While Macron hinted on Thursday at more talks led by the EU chief negotiator Michel Barnier, the EU has not dropped its insistence that the backstop stays in the Withdrawal Agreement.

As Merkel in particular emphasized, the non-legally binding Political Declaration on the future relationship between the U.K. and the EU — the second element of the Brexit deal — can be changed, and Barnier is authorized to start talking about that. But without changes to the Withdrawal Agreement the U.K. won't be at the table — so deadlock looks likely to persist.

EU leaders also have half an eye on Johnson's home front. MPs return to parliament in little over a week's time, and opposition parties and Conservative rebels are expected to fight tooth and nail to block Johnson from taking the U.K. out of the EU on October 31 without a deal. While it is not yet a public position, many government officials are convinced Johnson will seek an election to prevent the House of Commons — where he has a majority of one — standing in his way.

For now, that is where Johnson's real fight lies, and until he wins it, his fellow G7 leaders will be justified in wondering how long his tenure as a member of their exclusive club will be.

David Herszenhorn and Gabby Orr contributed reporting for this article.

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