PARIS — It has happened rarely between European Union allies, and not between France and Italy since World War II. But on Thursday, after months of barbed commentary from Italian leaders, the French government said it had had enough: It recalled its ambassador from Rome.

“This is without precedent since 1940, when Mussolini declared war,” said Marc Lazar, a specialist in Franco-Italian relations who teaches at universities in Paris and Rome. “This is very, very harsh. There’s never been anything comparable.’’

The protest not only demonstrated the breakdown of relations between France and Italy, founding members of the European Union. It also reflected the mounting strains at Europe’s core brought on by populists seeking to denigrate the bloc and forge anti-European alliances across borders, a clash that may play out even more bitterly in European Parliament elections in May.

But now that the populists run the Italian government, Europe’s divided politics have been elevated to the level of diplomatic rancor.