Ian Buruma is the author, most recently, of The Churchill Complex: The Curse of Being Special, From Winston and FDR to Trump and Brexit. When political leaders, and their admirers, claim that George Soros, the Hungarian-American-Jewish philanthropist, is pulling the strings of world affairs, we know that antisemitism is not far off. But the antisemitic nature of these claims has not stopped Hungary’s Prime Minister Viktor Orban and former U.S. president Donald Trump, along with their followers, from propagating them. Both Mr. Orban and Mr. Trump often point to their support for Israel as proof that they are not antisemites....