A year ago, Deborah Lipstadt, newly confirmed as the U.S. special envoy to monitor and combat antisemitism, attended a White House reception and reintroduced herself to President Biden as he passed by. “I know who you are,” Dr. Lipstadt recalled the president telling her. “And you have a big job.” Mr. Biden was right, but for reasons neither of them fully imagined. Dr. Lipstadt, whose role at the State Department for the first time carries the rank of ambassador, “leads efforts to advance U.S. foreign policy to counter antisemitism throughout the world,” according to her job description. But as she...