The Iran nuclear deal, cosigned by France, Germany and the United Kingdom, was hailed as a success for Europe’s style of multilateral diplomacy, so President Trump’s withdrawal from the agreement sent shock waves through the Continent’s capitals.

German Chancellor Angela Merkel said, for the second time in a year, that Europe could no longer rely on the U.S. to protect it. The president of the European Council, Donald Tusk, tweeted: “Looking at latest decisions of @realDonaldTrump someone could even think: with friends like...