ONE OF THE most tantalizing might-have-beens of the Second World War occurred in the summer of 1939 when Enrico Fermi, the brilliant Italian physicist who had recently emigrated to America, failed to persuade his visiting German counterpart, Werner Heisenberg, to join him on the physics faculty at Columbia University. Heisenberg chose instead to return to Germany, where he headed the Nazis' unsuccessful effort to build an atomic bomb. Fermi, some three-and-a-half years later, led the U.S. team of scientists that achieved the world's first self-sustaining atomic chain reaction -- the premier practical step toward the weapons that would eventually be dropped on Japan.

While the Germans never really got close to building a bomb -- a point MacPherson's book confirms -- so exalted was Heisenberg's reputation in America that as late as June 1944, Robert Oppenheimer, the Manhattan Project's chief scientist, wrote that if he and his colleagues were to try building the bomb in Germany "we should have made desperate efforts to have Heisenberg as collaborator. It is hard for us to believe that they would be embarked on this program without taking advantage of his help." Had Heisenberg decided to stay in America, therefore, the motivating anxiety that ultimately compelled Fermi and his friends to win the race for the atomic bomb -- the fear that the Germans might get it first -- would have vanished into air.

Time Bomb tells the story of that race, which has long been a subject of interest to historians but is only now becoming a favorite topic for novelists and writers of popular non-fiction. Fermi and Heisenberg are a logical choice for the centerpiece of the story, since their prewar careers essentially followed a parallel course -- the same age, with the same mentor, each would win a Nobel Prize before he was 40 -- though their personalities and personae were at opposite ends of the scale: Fermi diminutive, olive-skinned, and Jewish -- the self-effacing "Noble Roman" to his friends; Heisenberg a tall, blond physical epitome of the Third Reich's Aryan ideal, who immodestly told his mother upon winning the Nobel Prize, "Mama, I congratulate you on your son."

Relying upon secondary sources and interviews with those who knew the principals -- Fermi died in 1954, Heisenberg in 1979 -- MacPherson succeeds in recreating the sometimes breathless mood behind the accelerating and ultimately headlong rush of the world's scientists toward the bomb. Although Time Bomb's tale of the early atomic age covers ground already well-trodden by other recent books, MacPherson tells the story in more detail and with greater skill than most of his predecessors -- in part because he deliberately chose to concentrate upon the pivotal years 1938-42, when the outcome of the race for the atomic bomb was determined. MacPherson is also better at explaining the complex science behind the bomb in a manner that neither loses nor patronizes the non-scientist reader.

Missing, of course, from this treatment is the rest of the story -- the part, for example, that Fermi played as a scientific advisor to the Truman administration's Interim Committee in determining how what he and his colleagues had created would be used. In an epilogue, MacPherson notes that German scientists learned of the Manhattan Project only when they heard the news of Hiroshima on the radio while prisoners-of-war in England. Heisenberg, true to form, was initially disbelieving that his American and British rivals could have succeeded where he had failed -- attributing the destruction of the Japanese city to "some chemical substance with which they've enormously intensified the explosive force." The truth, however, was more readily apparent to his countrymen and erstwhile colleagues: "Anyway, Heisenberg," observed one brutally, "you're second-rate and can pack your bags." TIMEBOMB is essentially a morality play of good triumphing over evil. Somewhat disappointing, therefore, is its failure to reflect upon the more complex mixture of motives that guided Heisenberg's actions, for which the book presents ample evidence. Before the war, Heisenberg had been accused by the SS of being a "white Jew" -- "a Jew in spirit, inclination, and character" -- because he did not embrace the Reich's absurd "Aryan physics," which refused to recognize the contributions of Einstein and other Jewish scientists. Throughout the war, Heisenberg remained plainly disaffected toward the Nazis and ambivalent about the prospect of a German victory -- telling a colleague that, whichever side won, the postwar world "won't be a particularly good one either, and people will quickly realize that the war has solved few problems."

Without Heisenberg, Aryan physics alone would seemingly have doomed the Nazis to the loser's place in the race for the bomb, but MacPherson, like others, traces the fatal flaw in the German project to an early miscalculation which caused them to choose heavy water instead of graphite as a neutron moderator in their first atomic pile. In the eyes of some contemporary scientists, however, even this error fails to explain the uncharacteristically leisurely advance of German science in the atomic realm during the war -- and, significantly, MacPherson himself at one point concedes that it remains something of a mystery why Heisenberg neglected to check the relatively simple graphite calculation. A few of Heisenberg's defenders have even gone so far as to suggest that he deliberately slowed progress toward the German atomic bomb and to explain the physicist's visit to his mentor, Niels Bohr, in Nazi-occupied Copenhagen during the fall of 1941 as a successful effort to warn the Western democracies that Hitler was indeed intent upon having a nuclear weapon. MacPherson, on the other hand, accounts for Heisenberg's unprecedented breach of security simply as a failed attempt at gaining "absolution" from Bohr.

Between the lines rather than at the center of Time Bomb is a fascinating comment on the tension between a scientist's need to do science and his or her sense of pro patria. As MacPherson's book shows, the effect of this tension was evident even before the advent of the bomb itself -- in the failure of the international scientific community to keep their findings on fission research a secret once its military implications became clear. Simple ambition and the urge to publish before one's rivals confounded the naive hope that the bomb might be headed off before it reached the experimental stage. Whether Heisenberg deliberately dragged his feet on German atomic research or was simply tripped up by his own arrogance, he was at least ahead of his American and British rivals in recognizing the ethical dilemma that work on the bomb represented. Noting that the emigre' scientists in America "must all be firmly convinced that they are fighting for a just cause," Heisenberg, according to MacPherson, reflected during the course of the war:

"But is the use of an atomic bomb, by which hundreds of thousands of civilians will be killed instantly, warrantable even in defense of a just cause? Can we really apply the old maxim that the ends sanctify the means? In other words, are we entitled to build atom bombs for a good cause but not for a bad one? And if we take that view . . . who decides which cause is good and which is bad?." Gregg Herken is Senior Research Associate at the University of California's Institute on Global Conflict and Cooperation and author of "Counsels of War." He is currently writing a book on the president's science advisors.