Conspiracies about all-powerful Jews, like the Rothschilds and George Soros, offer a false sense of clarity for some Americans facing an increasingly traumatic society. The larger problem, historian Jonathan Sarna told a Washington, D.C.-area audience, is that you cannot disprove an argument rooted in fantasy. “If only we gave up Israel, or reshaped our noses, or gave up the Sabbath, antisemitism will disappear,” Sarna, a professor of American Jewish history at Brandeis University, said of the futile arguments Jews often make to themselves. “But antisemitism tells us much more about the antisemite than it does about Jews.” And in the...