The Guelph collection, a trove of medieval Christian art, was sold to Nazi-run Prussia in 1935. Was the sale fair, or did Goering make its Jewish owners an offer they couldn’t refuse?In October 1935, Prussian premier Hermann Goering proudly announced in a much circulated press release that he would soon present Adolf Hitler with a “surprise gift” — an extraordinary collection of medieval gold artifacts known as the Welfenschatz, or Guelph treasure. Forged of jewel-encrusted gold at the dawn of the second millennium CE, the dozens of Christian devotional masterpieces — ornate crucifixes, reliquaries of improbable detail, portable altars —...