Mr. Trump’s conflicting statements on China were far from the only puzzlements of his stay in Biarritz, France. He repeatedly touted what he said was a trade deal with Japan, only to be contradicted by Prime Minister Shinzo Abe and the Japanese foreign ministry, which said the negotiations were at a preliminary stage. He said there was “tremendous unity” in his talks with the other six leaders, though officials said the U.S. delegation blocked consensus on trade and other issues. Mr. Trump skipped a meeting on climate change, and his pitch to restore Russia to the group was flatly rejected by Germany and Britain, among others.

French President Emmanuel Macron made a valiant effort to use the summit to jump-start negotiations between the United States and Iran, even inviting Iranian Foreign Minister Mohammad Javad Zarif to Biarritz. Mr. Trump responded with more confusion: After allowing that Mr. Macron’s suggestion of a summit between him and Iranian President Hassan Rouhani within weeks was possible, he went on to cite conditions for a deal different — and less stringent — from those previously outlined by Secretary of State Mike Pompeo. Mr. Trump lambasted President Barack Obama for striking a deal that granted Iran economic concessions, then suggested that he would support new loans for Tehran if talks got underway.

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Mr. Rouhani suggested in a speech Monday that he was open to negotiations, so perhaps something will come from Mr. Macron’s initiative. But there was no way to judge from Mr. Trump’s remarks whether he was seriously contemplating a change of tack on Iran — just as it was anyone’s guess whether he had second thoughts about the trade war he started with China, as he suggested Sunday, or merely wished he had raised tariffs even higher, as his staff later said.

The one subject on which Mr. Trump’s intentions appeared unambiguous was his plan to steer the next G-7 summit, which the United States is due to host, to his own Doral golf resort near Miami — thereby injecting a huge stimulus into what has been a struggling business. When asked whether he was trying to use the presidency to enrich himself, Mr. Trump responded with the ludicrous claim that the presidency had cost him $3 billion to $5 billion. His scheme cries out for congressional intervention; if the emoluments clause of the Constitution means anything, it must forbid such blatant self-dealing.