In April 2016, at a campaign rally in Wilkes-Barre, Pennsylvania, a grinning Matteo Salvini – now deputy prime minister of Italy, and the leader of its far-right Lega party – photobombed another then-rising star of the populist right, Donald Trump.

For the social media-savvy Salvini, it was a brilliant piece of publicity. It also caught the eye of Steve Bannon. Within 48 hours, the Italian politician was in Washington DC, meeting with the head of the attack-dog Breitbart news network who soon took over Trump’s campaign.

Both Bannon and Salvini are now at the helm of grand plans to unite the right across Europe ahead of the European Parliament’s elections next month. Much of the conversation they had in 2016, before either had tasted triumph at the polls, remains a mystery.

But a senior Lega party insider with knowledge of the events that day, who spoke to investigative journalists at SourceMaterial on condition of anonymity, said that Salvini emerged from his meeting with Bannon with a key piece of advice: attack the pope.

Returning to Italy, Salvini did not take long to put this into action. “The pope says migrants are not a danger. Whatever!” he tweeted in May 2016. That summer, he was photographed holding up a t-shirt emblazoned with the words “Benedict is my pope”.

That slogan is a Vatican version of the “birther” campaign that Trump waged against Barack Obama: that irregularities in the conclave that elected Francis, the current pope, make his papacy illegitimate – and that his predecessor, Benedict XVI, remains the ‘real’ pontiff.

Meanwhile, Bannon – whose political vision centres on the defence of “Judeo-Christian values” in the West – has also become close to some of Francis’s most implacable foes in Rome, including the ultra-conservative Cardinal Raymond Burke.

Burke, who has feuded with Francis since the outset of his papacy, is part of a group of hardline critics who claim that “organised networks” of LGBT people are spreading a “gay agenda” in the Vatican that they say is the root cause of recent sex abuse scandals.

In 2017, Bannon became a patron of the Dignitatis Humanae Institute (DHI) ‘think tank’ – of which Burke is honorary president. Recently, his plans to use an ancient monastery outside of Rome as a ‘gladiator school’ for populists have prompted protests.

They have also shone light on the network of individuals and groups on both sides of the Atlantic who have been driving anti-Francis campaigns – bringing together anti-immigrant populists, anti-abortion activists, alt-right celebrities and Catholic fundamentalists.