The Origins of the Final Solution: The Evolution of Nazi Jewish Policy, September 1939-March 1942. By Christopher R. Browning, with a contribution by Jürgen Matthäus. University of Nebraska Press, 640 pages, $39.95.

In 1941 Walter Mattner, a low-level German police secretary stationed in Mogilev, Belorussia, wrote a letter home to his wife in Vienna. “When the first truckload [of victims] arrived my hand was slightly trembling when shooting, but one gets used to this,” Mattner wrote. “When the tenth load arrived I was already aiming more calmly and shot securely at the many women, children, and infants.”

How did the Nazis turn ordinary people into killers? In Origins of the Final Solution, history professor Christopher Browning examines how Nazi policy evolved during the first thirty months of World War II.

Hitler’s orders concerning Jews were intentionally vague, Browning says. “Hitler represented what I call an ideological imperative — he legitimized the necessity for finding a solution, but he did very little in terms of hands-on policy making. Various people came to him with various proposals, and he gave them the red light or the yellow light or the green light, but it certainly wasn’t a matter of orders from above and coercion. People vied with one another to get credit for being the ones offering the most radical solutions to the Jewish question. Hitler was in a sense at the switch, deciding which proposals to approve and who to give more power to and who to reward for what they had done before.”

In the war’s early years, proposals under consideration included expulsion — shipping all European Jews to Madagascar, for example. By the summer of 1941, the Nazis had moved from socially marginalizing Jews, to relocating them, to the active pursuit of the unthinkable: the murder of every last Jewish man, woman, and child. “The question,” Browning writes, “was no longer why the Jews should be killed, but why they should not be killed.”

Browning sees the so-called gas van as the first step toward that terrible conclusion. To cull those they considered genetically weak or inferior, the Nazis drove a van labeled “Kaiser’s Kaffee Geschäft” (Kaiser’s Coffee Company) through the German-annexed territories of Western Poland in 1939. The Nazis loaded the van with mental patients, pumped it full of carbon monoxide, and buried the bodies in the forest. By early 1940 the Nazis were killing mentally and physically handicapped Germans in stationary gas chambers. The gas chambers at Birkenau, the largest of Germany’s concentration camps, could kill thousands of people in a day.

It’s always easy to identify the Holocaust with Hitler, which is certainly not wrong,” Browning told the Atlantic Monthly. “He was, as I argue, the prime decision maker and instigator in this. But if we want a fuller picture of how these things came about, then we need to get at the layered, complex reality in which all sorts of people made incremental contributions.”

Killing innocent civilians may have made sense to people such as the police secretary Mattner, Browning continued, in part because they had been convinced they were protecting themselves. “First you divide people between ‘us’ and ‘them.’ Then you cast the other — ‘them’ — as a terrible threat. In turn, you justify your doing terrible things to ‘them’ as self-defense.”

Mattner rationalized his killing, writing, “I too have two infants at home, with whom these hordes would do the same, if not ten times worse.”

Browning says, “I would argue that many of the elements in this were a coming together of quite common factors and ordinary people. That, I think, is very important to recognize if we don’t want to place the Holocaust apart as some kind of suprahistorical, mystical event that we cannot fathom and shouldn’t even try to understand.”