In the early years, when he was still a secessionist who believed northern Italy should break off and become an independent state called Padania, Matteo Salvini held his fellow countrymen in such contempt that he even supported Germany over Italy in the 2006 World Cup. “Italy ... is the worst of the worst,” he said at the time. “My support goes to anyone who is more serious.” Today, the 45-year-old has adopted “Italians first” as a political ideology and is in many ways the European leader who most resembles Donald Trump, from his embrace of impulsive and often racist rhetoric,...