AT TREBLINKA, the Nazi extermination camp, he operated the engines that pumped carbon monoxide into gas chambers where over 750,000 jews perished. According to one camp surivior, "from time to time, he (got) an urge to take a sharp knife, stop a worker who (was) running by, and cut off his ear." To amuse himself, he ordered Finkelstein, a dentist who extracted metal fillings from the teeth of corpses, to lie down on the ground and drilled the metal into Finkelstein's anus. Although he was an ethnic Ukranian, he was called "Ivan the Terrible," after the 16th-century Russian czar who was both an anti-Semite and a sadist. Today, four decades later, he uses his real name, John Demjanjuk. He is 64 years old, and lives in a suburb of Cleveland, Ohio.

Quiet Neighbors: Prosecuting Nazi War Criminals in America by Allan Ryan, tells how the U.S. government allowed Demjanjuk and other active participants in Nazi genocide to immigrate after the Second World War and granted them a safe haven for over three decades. From 1979 to 1983, the author headed a special Justice Department unit that was created to prosecute former Nazi collaborators living in the United States. Ryan's book has significant drawbacks, notably the self-congratulatory descriptions of his own role, as well as the rambling travelogues of his official trips. Nevertheless, he provides an original and disturbing account of immigration policies that admitted ex-Nazis to the United States, policies that sacrificed moral awareness of Nazi war crimes to the prevailing preoccupation with communism.

This history is one thread in a larger fabric of official American indifference to the persecution of Jews and other minorities by Nazi Germany. In the late 1930s, European Jews fleeing Hitler were often barred from entering the United States. During the Second World War, the American government rejected pleas that it bomb gas chambers used daily to kill tens of thousands of Jews. After the war, the U.S. Army employed ex-Nazis who had directly participated in genocide, including Klaus Barbie, former Gestapo chief in Lyon, and scientist Arthur Rudolph, former head of a rocket factory where thousands died in forced labor.

Ryan's particular story begins at the end of the war when hundreds of thousands of Eastern Europeans -- Ukrainians, Yugoslavs, Romanians, and others -- sought refuge in displaced persons camps in Western Europe. Many had been Nazi collaborators, but were now posing as Nazi victims. In 1945 The New York Times reported, "thousands of former allies of Germany are being fed and housed in the camps," and estimated that "one-third and probably more" of the inmates had worked for the Nazis.

Congress, identifying Eastern Europeans in the camps as devoted anti-communists because of their unwillingness to return to their Soviet controlled homelands, enacted the Displaced Persons Act of 1948 to give them an immigration preference. While former Nazi collaborators technically were prohibited from immigrating, no provision was made for verifying who was an ex-Nazi and who was not. Thus, Demjanjuk and others like him were able to enter America by concealing their Nazi past.

There were repeated warnings that the law would permit Nazi collaborators to enter the United States. But in 1948, when the Displaced Persons Act was passed, Nazism had been defeated and the new menace was Soviet communism. For "those who opposed communism, the path to America was open" even though careful investigation might have shown they had worked for the Nazis, writes Ryan.

IRONICALLY, the same immigration law that effectively admitted Nazi collaborators also attempted to exclude Jews. Immigration quotas favored Eastern European ethnic groups (such as Latvians), with virtually no surviving Jews, as well as occupations (such as farming) where few Jews were employed. One Senate sponsor commented that the Act would thereby help ensure that every new immigrant was not "related to residents of New York City."

Once in the United States, Demjanjuk and others like him took jobs, raised families, and generally prospered. After the five-year residency period required by law, they applied for and became naturalized American citizens.

In a handful of cases, Ryan notes, their presence in the United States was discovered by Holocaust survivors, the U.S. government was alerted, and the Immigration and Naturalization Service tried to deport them. Among those identified were Andrija Artukovic and Viorel Trifa. Artukovic, minister of the interior in the puppet Nazi state of Croatia, signed orders establishing concentration camps where internal security forces under his authority killed hundreds of thousands of Serbs and Jews. Trifa, a leader of the Iron Guard, a Romanian fascist, anti-Semitic movement, incited riots in which hundreds of Romanian Jews were killed.

However, U.S. officials did not consider the presence of such individuals to pose any special problem, and the INS lost most deportation cases through indifference and incompetence. In the case of Artukovic, one of the few which INS actually won, the deportation order was never carried out. The deputy attorney general of the United States, Peyton Ford, recommended that Artukovic be allowed to stay because "his only crime was against communists."

Not until 1979, under growing pressure from American Jews, did Congress create a special unit in the Department of Justice, the Office of Special Investigation (OSI), with responsibility for prosecuting ex-Nazis living in the United States. By this time, however, many had died. Even if alive, they could not be tried in American courts for participating in genocide, which is not a crime under American law. The only legal actions available were the highly cumbersome ones of denaturalization and deportation: strip them of their U.S. citizenship, on the ground that they entered the United States by concealing their Nazi past, and then deport them.

In 1979 OSI began to proceed against former Nazi collaborators living in the United States. Altogether, five have been deported, and cases are pending against 35 others. Although Trifa voluntarily agreed to be deported in 1980, it took four years to find another country willing to admit him. A federal court has ruled that Artukovic is entitled to a new deportation trial, expected to start next year. Demjanjuk remains in Cleveland while his deportation order is on appeal.

OSI's activities pose a moral dilemma, which Ryan recognizes with the question, "why disrupt the lives of people who have lived among us for more than three decades . . . in their sixties and seventies . . who pose no present danger to anyone. . .?" Ryan's answer is to prevent another Holocaust. He does not discuss vengeance as a possible justification, perhaps because revenge is often considered irrational or barbaric and Ryan is trying to present OSI in the best possible light.

Yet troubling as it is, vengeance was how the philosopher Hannah Arendt justified punishing Adolph Eichmann, who administered the Nazi concentration camps. I wish that Ryan had quoted the closing of Eichmann in Jerusalem, Arendt's book on Eichmann's trial and execution by Israel in the early '60s:

"Let us assume, for the sake of argument, that it was nothing more than misfortune that made you a willing instrument in the organization of mass murder; there still remains the fact that you have carried out, and therefore supported, a policy of mass murder. For politics is not like the nursery; in politics, obedience and support are the same. And just as you supported and carried out a policy of not wanting to share the earth with the Jewish people and the people of a number of other nations -- as though you and your superiors had any right to determine who should and should not inhabit the world -- we find that no one, that is, no member of the human race, can be expected to want to share the earth with you."