Diplomatic etiquette would normally classify the recall of an ambassador for “consultations” as a middle-order symbol of displeasure. During the cold war, the summoning, or withdrawal, of an ambassador was mundane. More recently, Hungary pulled its ambassador from the Netherlands in 2017, in response to criticism by the outgoing Dutch ambassador in Hungary.

But for France to withdraw its ambassador to Rome for the first time since the second world war represents a genuine diplomatic shock. For two European powers to fall out to this extent shows how far Italy’s government of the far-right League and anti-establishment populist Five Star Movement (M5S) is prepared to break the rules.

Only a fortnight ago, faced by persistent insults from Rome, the Elysée chose to take the high road, saying it would not enter a stupidity contest. President Emmanuel Macron had also promised not answer back, saying that is what the Italians wanted.

But faced by Italian deputy prime minister Luigi Di Maio’s repeated courting of leaders of the gilet jaunes (yellow vests) protests that have repeatedly sparked violence in Paris, French patience snapped.

It marks an extraordinary collapse in Franco-Italian relations since the recent high water mark of January 2018 when Macron signed a bilateral treaty of friendship alongside Italy’s previous prime minister, Paolo Gentiloni. That was only two months before the Italian elections in May.

Macron had signed the treaty partly to reassure the Italians that Paris would not only face toward Berlin after Brexit.

But perhaps the seeds of the collapse were sown the day the treaty was signed. In Rome, Macron could not resist saying he hoped the Italians in their elections would make a pro-European choice – advice that Italians, fixated by migration from Libya, totally ignored by bringing a League and M5S coalition government to power.

The two parties felt Macron had tried subtly to keep them from office, and now think the remote former banker deserves to suffer for his misreading of the public’s anger with the elite. Italian anger at France’s refusal to take its share of migrants infuriates Salvini.

But tweaking the nose of the French president also allows the two Italian deputy prime ministers – Luigi Di Maio and Salvini – to signal to their supporters they are made from a different mould from the old political class. They ignore the normal courtesies, and in no field are there more courtesies than diplomacy.

Some of the dispute is about a struggle for national interest, including for influence in Africa. In Libya, the two countries vie with one another to lead European diplomacy, holding rival summits. Di Maio decries French colonialism. In Venezuela, Salvini encouraged by Russia, rejects French interference in the affairs of a Latin American country.

But the true context for the row is May’s European elections. Both Macron and the Salvini-Di Maio partnership are using each other as foils to define their campaigns and mobilise their core vote. Di Maio claims the yellow vests are a sign that the winds of change are sweeping across the Alps, and Macron, a bad president, will as a result be defeated, leading to broader changes in Brussels and the European parliament.

The danger is that the stream of insults pouring off the Italian party leaders’ Facebook pages has not just political but also economic consequences, with Italy coming off worse. Trump can insult with impunity since he heads a superpower. Italy, by contrast, is less powerful, and an alliance with Viktor Orbán’s Hungary is hardly a sufficient shelter.

Already some businesses that co-operate across the Franco-Italian border are feeling uneasy. Alitalia is hoping for French capital to save the airline. Infrastructure plans such as the Turin-Lyon high-speed rail link are in doubt.

Italy, in recession and heading for only 0.2% growth this year, will need some allies in Europe and in Brussels. Its banking system remains undercapitalised. M5S is determined to show it is on the side of the people, and not the bankers, but translating that emotion into practical budgetary policy is proving difficult. Insults by contrast come easier, and cheaper.