It was one of the most ruthlessly conceived and executed invasions in the annals of warfare. Hitler's Polish campaign unleashed a blitzkrieg in which SS troops, police squads, and the army itself waged an ethnic war of unprecedented brutality. Tens of thousands of Poles--roughly 80 percent of whom were Christian--were summarily executed in acts of collective punishment. After six weeks, a country was crushed and the world was at war.

Usually given short shrift in most histories of World War II, the invasion of Poland was more than a series of opening salvos; it was a testing ground for...