In the winter months of 1939-40, after the savage destruction of Poland, all was quiet on the western front in Europe. Hitler’s Panza tanks and Stuka dive bombers were silent. Stalin was busy chewing off bits of Finland in the Winter War, but Hitler was biding his...

Eighty years ago this month, at the beginning of April 1940, Europe was enmeshed in an uneasy calm. Hitler had launched World War II the previous September, when he fell upon Poland with all the fire and fury of Blitzkrieg—‘lightning war.’ As he tore through that...