THE POLK CONSPIRACY Murder and Cover-up In the Case of CBS News Correspondent George Polk By Kati Marton Farrar Straus Giroux. 369 pp. $22.95

THE MURDER in Greece in 1948 of CBS correspondent George W. Polk is an eerily appropriate mystery to be solved with the disappearance of the Cold War. Anyone who wants to understand the moral wilderness the United States entered when it began dealing with the world in red and white absolutes will find it all in microcosm in this riveting work, The Polk Conspiracy.

Greece was then engaged in a civil war, the right-wing monarchist government on one side and Communist guerrillas on the other. A parliamentary monarchy had been restored soon after World War II, through strong British support. Britain's interest was understandable. Greece occupied a critical location on the Mediterranean route through Suez, Britain's lifeline. Certainly no place to have a Communist regime mucking about. But by 1947, a nearly bankrupt Britain could no longer afford to provide the Greek government much aid in its war against the Communists. The Truman Administration had just developed its response to the spreading Soviet stain, the policy of containment. And in the first Cold War test of this "Truman Doctrine" the U.S. stepped in and took over the burden of underwriting the Greek civil war. Never mind that the restored Greek king was discredited and the incumbent regime corrupt and reactionary. For alongside the policy of containment came its corollary, "The Good SOB": They may be SOBs, but they're our SOBs.

Into this quagmire stepped George Washington Polk, CBS's chief Middle East correspondent. Polk was fearless, hard-driving and impatient, useful qualities in a journalist; dangerous qualities in the Greece of 1948. He made up his mind quickly about the Greek government: "It's fascist and below the belt in every way." Polk was branded a Communist in the Greek press. American diplomats in Athens questioned his patriotism. Here was an American attacking the government his country had chosen to support in the first test of the Truman Doctrine.

In the spring of 1948, Polk began making contacts to try to report from the Communist guerrillas' mountain fastness. At about the same time, he discovered that the Greek foreign minister, Constantine Tsaldaris, had illegally stashed a then substantial $25,000 in the Chase Bank in New York at a time when his country was trying desperately to stem the outflow of hard currency. Polk went to Tsaldaris with what he knew and threatened to "blow this story sky high."

Three days later, George Polk's body, bound and gagged and with a bullet hole in the back of his head, was fished out of the bay at Salonika. The Greek government had an instant explanation. Polk had gone to rendezvous with the Communist guerrillas, and they had killed him. Nevertheless, separate investigations were launched by the American government, CBS and a committee of "overseas writers" headed by Walter Lippmann. Among this cast, the author identifies American villains, heroes and dupes. For the Polk case is more than a tale of covered-up murder. In Kati Marton's skilled hands it becomes a parable of the endless struggle between the truth and political expedience.

Walter Lippmann had made a seemingly sterling choice to carry out the overseas writers' investigation, Gen. William J. "Wild Bill" Donovan, the father of the CIA's predecessor, the Office of Strategic Services. Donovan turned over the day to day digging to James Kellis, an OSS veteran. Kellis, tough-minded and relentless in exploring gaping holes in the Greek government's story, emerges as the hero: Marton's villain is Donovan, who ignored Kellis's growing evidence that Polk was rubbed out, not by the Reds, but by the Greek right. To Donovan, identifying Polk's killers was less important than the danger of discrediting an anti-Communist ally. Polk was merely a civilian casualty of the Cold War.

In due course, the Greek police produced a suspect, Gregory Staktopoulos, an obscure Greek journalist with Communist connections. And, according to Marton, they tortured a confession out of him. Staktopoulos was tried and convicted. Donovan embraced the verdict as a tidy end of the matter.

The unexpected dupe, in Marton's telling, is the respected Lippmann, who mouthed virtually verbatim the Greek government's line: The Polk killing was supposedly engineered by Moscow in order to put the blame on the Greek government, thereby tarnishing it in the eyes of America and thus ending Marshall Plan aid to Greece. To accept this pretzel reasoning one would have to believe that the Communists would kill a broadcast journalist whose reports criticized their enemy and who was trying to report from their side of the war.

A reader hopes, on picking up a book dealing with a clouded, 40-year old murder, to find at last the definitive solution. Marton does give a chilling description of how Polk was killed by agents of an extremist right-wing group with which Foreign Minister Tsaldaris was evidently associated. Yet, while all of her other sources are meticulously identified, the source of this key disclosure, the one that nails down her case, is not clear. We don't know exactly how she knows. And, in all fairness, it must be pointed out that, along with Donovan and Lippmann, the CBS team investigating the murder, the New York Times correspondent and other American observers accepted the Staktopoulos verdict. Marton's circumstantial case is thus stronger than her factual case. Still, it is convincing. The Communist guerrillas had no motive to kill Polk. Green right-wing extremists did.

Marton could also have written a more compelling book by being less obviously partisan. A bit more reportorial detachment would have allowed the reader the pleasure of making up his or her own mind. We would still likely come down on her side, but persuaded by the weight of evidence rather than being instructed.

Nevertheless, Marton has written a harrowing real-life thriller and an important morality tale. She has taken the tragedy of one journalist and used it to illuminate an American quandary: What happened when America bedded down with unsavory regimes whose only virtue was that they were anti-Communists? What happened when America forced its foreign policy into a rigid matrix of East against West, Communist versus anti-Communist? Kati Marton provides her own answer. "The road from Athens led ultimately to Saigon," she says. And George Polk was a truth-telling voice suppressed early in this journey. Joseph E. Persico is the author of the forthcoming "Casey: The Life and Secrets of William J. Casey from the OSS to the CIA."