Given all this, the European Union in particular may feel compelled to acknowledge the harsh reality that it has yet to act like an organization with a G.D.P. roughly equal to that of the United States — and to change the situation by signaling to Washington that on certain matters it refuses to be ignored. The trouble the European Union has had uniting around highly sensitive issues — immigration, for example — is perhaps understandable. But the Iran deal does not appear to be divisive: From the pro-Brexit Mr. Johnson to the avowed internationalist Mr. Macron, European politicians understand its value.

What can Europe do to get Mr. Trump’s attention? President Hassan Rouhani of Iran has indicated that his country will continue to adhere to the terms of the deal for the time being and deal with other parties to it in hopes of securing its benefits. This confers on the European Union, Britain and France — all of which are parties — some leverage. Acknowledging Iran’s compliance and continuing to do business with Iran without imposing sanctions would mitigate the effect of the White House’s sanctions and make it easier for advocates of the deal in Tehran to make their case.

The administration’s challenge — the American ambassador to Germany has already said that German companies doing business in Iran should wind down operations now — might prompt Europe to go further. The European Union could, for instance, announce the withdrawal of member-states’ ambassadors from the United States. Isn’t this what states do when diplomatic partners breach solemn agreements, expose them to security risks and threaten to wreak havoc on their economies? That is, after all, what the administration is threatening to do by courting the risk of a Middle Eastern war and applying secondary sanctions to European companies. Depending on the American response, European capitals might even follow up with expulsion of American ambassadors.