BAD AROLSEN, Germany - Within weeks of Hitler's 1933 rise to power, the iron gates slammed shut on inmates of the first Nazi concentration camps. It was the start of an unparalleled experiment in persecution and genocide that expanded over the next 12 years into a pyramid of ghettos, Gestapo prisons, slave labor camps and, ultimately, extermination factories. Holocaust historians are only now piecing together the scattered research in many languages to understand the vast scope of the camps, prisons and punishment centers that scarred German-ruled Europe, like a pox on the landscape stretching from Greece to Norway and eastward...