The United Nations and many countries officially commemorate the Holocaust on Jan. 27, the anniversary of the liberation of Auschwitz. That place, where the Nazis perfected mechanized murder, has correctly become a universal symbol of the genocide of the Jewish people. But the Holocaust did not begin at Auschwitz. Rather, 70 years ago, well before the gas chambers and crematoria were designed or constructed, the systematic murder of Europe’s Jews began as the German army swept through Poland, Ukraine and Belarus on its way eastward. Mobile killing units, frequently aided by regular soldiers and local collaborators, shot them where they lived and buried their bodies in hundreds of mass graves.