Already, Mr. Macron spoke by telephone on Wednesday with his Iranian counterpart, Hassan Rouhani. Afterward the Élysée Palace issued a statement saying that it was “the will of France to continue to enforce the Iran nuclear deal in all its dimensions,” potentially widening a breach with the Trump administration.

“He pointed out the necessity for Iran to do the same,” the statement added.

But the real question for the Europeans, Mr. Shapiro said, “is not if they stick with the deal but will they stand up to the American effort to unravel it and take active measures to protect their companies and banks trading in Iran?” That would be “an extremely confrontational stance,” he said, “and it’s not clear that their companies really want that.”

While some think that they should double down on what has now become a pattern — keep talking to Mr. Trump and his aides, hoping to convince them of the need for trans-Atlantic solidarity — others have had enough.

There are increasing voices for rupture within the European Union. In a reflection of the mood, Jean-Claude Juncker, the president of the European Commission, the bloc’s bureaucracy, said that under Mr. Trump, the United States is turning its back on multilateral relations and friendly cooperation “with a ferocity that can only surprise us.”

He told Belgium’s Flemish regional parliament that Washington “no longer wants to cooperate with other parts in the world,” according to The Associated Press. “At this point, we have to replace the United States, which as an international actor has lost vigor, and because of it, in the long term, influence,” he said.

In Britain, Emily Thornberry, the Labour Party spokeswoman on foreign affairs, said on Tuesday that it was time for Europeans to stop “this long and unnecessary indulgence of Donald Trump.”

A senior adviser to the European Union, Nathalie Tocci, said that the Iran deal was a lost cause, because “Trump and Europe have fundamentally different objectives.”