After months of swaggering hesitation, President Trump finally announced the United States’ withdrawal from the nuclear deal with Iran, to which Britain, France, Russia, China, Germany and the European Union are also parties. This action tramples on European leaders, who urged Mr. Trump to exercise restraint in the interest of international security and multilateralism.

Federica Mogherini, the European Union’s high representative for foreign affairs and security policy, reacted to Mr. Trump’s announcement with a sharp statement. The European Union and the rest of the international community, she said, would “preserve this nuclear deal.” The question is how. Notwithstanding an abundance of kvetching, European powers have not yet shown Mr. Trump that he has anything to fear from ignoring their wishes.

To be sure, European leaders have expended plenty of verbiage, illuminating the broad consensus that Iran is abiding by the terms of the deal. President Emmanuel Macron of France implored the Trump administration not to scupper the agreement during a recent visit to Washington. Chancellor Angela Merkel of Germany echoed his sentiments, as did Britain’s foreign minister, Boris Johnson, in these pages on Sunday.