By Rebecca Ballhaus, Noemie Bisserbe and Max Colchester 

BIARRITZ, France -- President Trump clashed with world leaders over the U.S. trade war with China and a host of foreign-policy issues at a Group of Seven summit that showed his isolation on the world stage.

A weekend of meetings at this sea resort included an unexpected visit by Iranian Foreign Minister Javad Zarif at France's invitation that caught some U.S. officials off guard.

Mr. Zarif, who was sanctioned by the U.S. last month, met with French President Emmanuel Macron and other French officials and briefed the U.K. and German delegations. Hours later, he left Biarritz without meeting U.S. officials.

Throughout the summit, attempts at bonhomie -- Mr. Trump kissed German Chancellor Angela Merkel on both cheeks -- gave way to tensions. G-7 leaders tried to squeeze concessions from Mr. Trump on Iran and other issues over closed-door meals, beyond the reach of White House advisers and TV cameras.

But Mr. Trump responded by doubling down on his policies and offering accounts of group meetings that conflicted with other countries' descriptions. He reaffirmed his commitment to a trade war with China and asserted his right to declare a national emergency over the matter; contradicted other leaders and his own administration over whether a decision had been made to invite Russia to next year's gathering; disputed French officials who said the leaders had reached a consensus over how to deal with Iran; and broke with Japan over whether recent missile tests by North Korea constituted a violation of agreements.

"We're in the world of missiles, folks, whether you like it or not," Mr. Trump said.

And a Sunday breakfast that was supposed to showcase Mr. Trump's budding alliance with new British Prime Minister Boris Johnson cooled as the U.K. leader warned the president about the effects of climate change and questioned Mr. Trump's pursuit of a trade war.

"We don't like tariffs on the whole," Mr. Johnson said.

At one point, Mr. Trump suggested he was having "second thoughts" about the escalating trade war with China -- comments that were initially viewed as a concession of sorts by the president toward the other leaders attending the G-7, who have called for an end to the trade war and say it is damaging the global economy and weakening alliances.

"I have second thoughts about everything," Mr. Trump told reporters as Mr. Johnson looked on.

Hours later, however, White House press secretary Stephanie Grisham issued a statement saying the president's answer had been "greatly misinterpreted" and that he regretted only that he hadn't "raised the tariffs higher."

Mr. Trump's decision to dig in on the trade war risks further rattling global markets. Investors worry a deepening of the dispute will dim global growth and pave the way for a recession. The escalation in tariffs from both China and the U.S. last week sent stocks and government bonds tumbling.

The continued tension among longtime allies and trading partners is also a measure of the breakdown in multilateral cooperation that has helped navigate past problems.

The summit began on a discordant note immediately after Mr. Trump's arrival on Saturday. Mr. Macron intercepted the president at his hotel and invited him to an unscheduled lunch without any of Mr. Trump's aides present.

For two hours, Mr. Macron explained his position on contentious issues including trade and the French leader's diplomatic efforts to de-escalate tensions with Iran, according to a senior French official. That encounter, the official said, laid the foundations for France to invite Mr. Zarif to Biarritz.

The lunch appeared to rankle U.S. officials who privately criticized Mr. Macron, accusing his government of ignoring Washington's input ahead of the summit and of focusing its agenda on "niche issues," such as climate change and development in Africa, to appeal to the French president's political base.

U.S. and French officials then bickered over whose idea it was to hold a session Sunday morning focused on the economy and trade. Draft agendas viewed by The Wall Street Journal showed such a session existed in June but was nixed or renamed in a version given to the White House in July. The U.S. said the session was added at its request, accusing France of not placing appropriate focus on the economy, though European officials disputed that description of events.

At a closed-door dinner Saturday evening, the conversation quickly turned to whether Russia -- which was expelled from the group in 2014 over its annexation of Crimea -- should be allowed to re-enter. The debate turned heated, according to one European official. While France, Germany and the U.K. had resisted Russia rejoining the group, Mr. Trump had called for Moscow's reinstatement. Mr. Trump's efforts were rebuffed, and the leaders agreed it was too soon to allow Russia back into the G-7, according to a U.S. official. Mr. Trump later said no decision had been made.

Also on the dinner agenda was the question of how to approach Iran, a matter that Mr. Trump and Mr. Macron have tangled over in recent months. According to an aide to Mr. Macron, the leaders agreed on the need to prevent Tehran from acquiring nuclear weapons and to find a path for Iran and the West to hold talks and ease tensions. The leaders then tasked Mr. Macron with delivering that message to Iran, the aide said.

Mr. Trump said he had discussed no such thing, and a person familiar with the matter said the president agreed neither to a joint message nor that Mr. Macron would be the messenger.

Later Saturday evening, Mr. Macron decided to invite Iran's foreign minister to Biarritz, according to the senior French official. The move was a huge gamble for Mr. Macron, who has been pressing the U.S. to soften its sanctions on Iran as a confidence-building step for further dialogue. The official said France felt it was important to "take stock of the situation" with Mr. Zarif.

On Sunday morning, the U.K.'s Mr. Johnson was filmed congratulating Mr. Macron on navigating the Russia tensions during the dinner.

"Well played," Mr. Johnson said to Mr. Macron in French.

Later in the day, as senior U.S. officials addressed reporters on a terrace overlooking the ocean, internet flight trackers noted that a jet carrying Mr. Zarif had touched down in Biarritz.

Mr. Macron met with Mr. Zarif at Biarritz city hall, and Ms. Merkel later described the Zarif meeting as a "parallel event" to the G-7 summit, adding that the meeting's organization was "closely coordinated" with Germany and the U.K.

The White House wasn't given an official heads-up about Mr. Zarif's arrival, but a person familiar with the matter said the president was aware the Zarif visit was a possibility. Mr. Trump, asked about the surprise arrival, gave a terse no comment.

--Valentina Pop in Brussels contributed to this article.

Write to Rebecca Ballhaus at Rebecca.Ballhaus@wsj.com, Noemie Bisserbe at noemie.bisserbe@wsj.com and Max Colchester at max.colchester@wsj.com

(END) Dow Jones Newswires

August 25, 2019 20:27 ET (00:27 GMT)

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