A secret history of the United States government’s Nazi-hunting operation concludes that American intelligence officials created a “safe haven” in the United States for Nazis and their collaborators after World War II, and it details decades of clashes, often hidden, with other nations over war criminals here and abroad. The 600-page report, which the Justice Department has tried to keep secret for four years, provides new evidence about more than two dozen of the most notorious Nazi cases of the last three decades. It describes the government’s posthumous pursuit of Dr. Josef Mengele, the so-called Angel of Death at Auschwitz,...