When the German Wehrmacht swarmed across Eastern Europe, an elite corps followed close at its heels. Along with the SS and Gestapo, the Ordnungspolizei, or Uniformed Police, played a central role in Nazi genocide that until now has been generally neglected by historians of the war.

Beginning with the invasion of Poland, the Uniformed Police were charged with following the army to curb resistance, pacify the countryside, patrol Jewish ghettos, and generally maintain order in the conquered territories. Edward Westermann examines how this force emerged as a primary instrument of...