In early 1939, peace activist Clare Hollingworth arrived on the Polish-German border to aid Jews and other refugees fleeing from the Sudetenland, newly annexed by Nazi Germany. On a brief return to her native England, the 27-year-old Hollingworth — who once professed to “enjoy being in a war” — was hired as a part-time correspondent in Katowice, Poland, for the London Daily Telegraph. After three days on the job that August, the cub reporter landed one of the biggest journalistic scoops of the 20th century: Hitler’s imminent invasion of Poland, marking the outbreak of World War II. Ms. Hollingworth, who...