In June 1940, Aristides de Sousa Mendes, Portugal’s consul general in Bordeaux, France, watched from his office window as a stream of Jewish men, women and children flooded his sidewalk. Hitler’s army had conquered France with shocking speed, and the Jews, now made stateless by Nazi racial laws, were at the consulate to plead for transit visas to Portugal—and to freedom. Sousa Mendes wrestled with their desperate appeal. His prime minister, António de Oliveira Salazar, had ordered him to deny all such requests, trapping the refugees in place as targets for Nazi arrest and deportation to concentration camps. Sousa Mendes...