World War II and the Nazi phenomenon maintain a peculiar fascination for the American imagination four decades later. Hardly a publishing season goes by without one or more best sellers on the subject, preferably now on the spy side. Is it because that war represented our last incontestable victory or because the roles of hero and villain were so clearly marked out -- or at least seemed to be?

Walter Murphy's second novel has all the ingredients of a genre success (the war, spies, a certain amount of sex and violence) with a few special twists that are also good box office (Vatican intrigue, flawed but sympathetic priests and a couple of non-Nazi officers). But Murphy also raises the ante a bit by introducing a moral dilemma that sets Americans against Americans and questions the easy assumption of total moral superiority that colors so much of the popular presentations of our involvement in World War II.

Indeed the plot turns on this dilemma and, as the novel slowly picks up speed, on a number of collateral ones that force the main characters to determine their fates apart from the machinations of their political masters. The most important and unpleasant of the latter on the American side is John Winthrop Mason, a man whose physical ugliness ("His facial features seemed the work of a genetic disaster or a misanthropic plastic surgeon") is matched only by his moral ruthlessness.

To convince the Germans that the Allies have not broken their Enigma code, Mason is willing to sacrifice as many lives as seem necessary on a series of fake missions to steal plans for the encoding machine. In this game of double deception everyone is expendable; in fact, the success of the operation depends on having the Allied agents captured and interrogated to underscore the seriousness of the effort to crack the already cracked code. What Mason leaves out of his elegantly contrived scenario are the moral scruples of the actors.

Chief among these is Fitzpa draig Cathal Sullivan, S.J., aka Father Christmas (F.C.), a prote'ge' of New York's Archbishop Spellman in the Vatican and an American spy in Nazi-occupied Rome. The Irish spelling seems a bit overdone for an American Irish Catholic of that era, but the combination of piety, wit and vanity in his makeup rings true. Which of that trinity will triumph in the end, as Sullivan begins to suspect what's really going on, becomes the novel's central question. (Mason bets heavily on the last.)

But Sullivan is not the only character afflicted with ethical qualms. The naval officer picked to be the scapegoat in Rome, Lt. Roberto Rovere, finds it necessary to corrupt a young Italian in order to blackmail a homosexual German officer with access to Enigma. When the boy commits suicide, Rovere confesses his self-loathing to Sullivan, who is thereby confirmed in his conviction that Rovere is sincere. This in turn frustrates an official attempt to get the Jesuit back in line by discrediting Rovere as a double agent.

Finally, there is an unnamed German Augustinian monk who teaches moral theology while moonlighting, like Sullivan, as a spy for his country. His love of Germany combined with his hatred and fear of communism allows him to support the Nazi's war aims and to deny as Allied propaganda all reports of the Holocaust to which he counters the undenied British saturation bombing of German cities. But, also like Sullivan, the monk sees himself as primarily a priest, and when confronted by a Vatican colleague with overwhelming evidence of the extermination camps, he acts symbolically but decisively to make atonement.

For all his interest in the psychology of clerics, Murphy is no Georges Bernanos ("The Diary of a Country Priest," and other novels). Like Murphy's best-selling "The Vicar of Christ," "The Roman Enigma" is primarily an action book with philosophical and theological set pieces inserted to lend depth. Since this is a much more streamlined novel than his first, there are fewer such digressions, but they do lend the narrative an air of erudition. More to the point of the genre, Murphy manages to work into his central Enigma plot the account of the infighting between the German Abwehr and the SS as well as a climax built around the Nazi roundup of the Jews of Rome. The former is particularly useful, for it allows him to redouble his original irony by having the Germans plotting to let the Allies steal a phony version of Enigma. Intrigue and counterintrigue are the name of this particular game, and the cast of more than two dozen characters (handily listed and identified in a pre-note) is kept busy spinning and cutting the intertwined webs.

But what raises this novel above many another World War II yarn is the way Murphy combines political realism and religious idealism to question the deepest ideology of all, a blind nationalism that justifies all excesses in the name of the greater good. By including an unsympathetic cameo of an indecisive Pius XII, Murphy extends his critique to ecclesiastical policies as well, implying that concern for the survival of the institution can lead to moral myopia. But Sullivan and his Augustinian counterpart offer some measure of redemption. Their faith, however shaky or compromised, allows them to transcend tribal loyalties; and their joint act of self-sacrifice, like Portia's "little candle," shines as a good deed in a world far more depraved even than that of the Jew-baiting Venetian merchant.