"I suppose I was courageous,” Muriel Gardiner reflected in an interview decades later. The American was living in Vienna in March 1938 when the Nazis took control of Austria. Muriel had already been part of a clandestine resistance group opposing Austria’s fascist dictatorship. But now, the country’s Jews and members of the resistance faced life-or-death danger. As many of the city’s residents poured into the streets, welcoming the installation of Nazi flags on storefronts and buildings, Muriel watched in horror. As an American, she could have easily packed up and moved home, leaving Europe’s problems behind. Plus, Muriel came from a life of privilege, having inherited millions at the age of 11. But before fleeing Nazi Vienna, she smuggled forged papers and sheltered Jewish strangers in her apartment. Thanks to her wealth and connections, Muriel also helped dozens of Austria’s Jews and opponents of the Nazis escape the Gestapo’s clutches. After the war, she sent monthly care...