To coordinate that effort, Mr. Garland appointed Eli Rosenbaum, a veteran prosecutor, in June to oversee the Justice Department’s war crimes accountability efforts. The choice was well-received: Mr. Rosenbaum is best known for his dogged pursuit of Nazi war criminals and the unmasking in the 1980s of the former U.N. Secretary General Kurt Waldheim’s role in the mass killings of civilians during World War II. Mr. Rosenbaum’s selection came as a surprise to him — he was on the verge of retirement — and he was immediately struck by the magnitude of the task. The prosecutor general’s office, Ukraine’s equivalent...