Since its beginnings, Poland has been a moving target, geographically as well as demographically, and the very definition of who is a Pole has been in flux. In the late medieval and early modern periods, the country grew to be the largest in continental Europe, only to be later wiped off the map for more than a century. The Polish phoenix that rose out of the ashes of World War I was obliterated by the joint Nazi-Soviet occupation that began with World War II. The postwar entity known as Poland was shaped and controlled by the Soviet Union. Yet even under these constraints, Poles persisted in...