IN 1946, as the Cold War dawned, the Soviet spy Julius Rosenberg helped to steal the first nuclear secrets from the most closely guarded place on the planet, the Los Alamos atomic research site in the New Mexico mountains.
With the material that Rosenberg, his wife, Ethel, and their ring were able to pass to Moscow in the 1940s, the Soviet Union began a nuclear programme that set the scene for the superpower confrontations of the next half-century.
Now, more than 50 years after the Rosenberg case and a decade after the Cold War was pronounced dead, the United States is struggling to face the possibility that an alleged spy called Wen Ho Lee may have done just the same on China's behalf. Experts indignantly deny it, but the fear is that the most closely guarded US nuclear secrets have been stolen in an effort to arm the nation that is seen as Washington's potentially greatest enemy.
Wen Ho Lee was born in Taiwan at around the time that the world's leading atomic scientists began work at Los Alamos on the Manhattan Project, the ultra-secret research and development programme that spawned the world's first atomic weapon. Like the original head of the Manhattan Project, J. Robert Oppenheimer, Lee is a scientist and a researcher at the University of California.
Though the details of his career still remain sketchy, Lee has worked at the US national laboratories at Los Alamos for more than a decade, along with his wife, Sylvia. Lee's chief expertise, sources say, is in the cutting-edge technology of computer-designed nuclear programmes, an area of research that earned him top security clearance and access to the crown jewels of the US nuclear weapons research programme.
Some time in the mid-1980s, if the allegations against him are true, Wen Ho Lee acquired detailed knowledge of the US's nuclear weapons miniaturisation technology, the knowhow that enabled Washington to develop a modern system of multiple miniature nuclear warheads.
Whether and how Lee passed these secrets to Beijing remains the subject both of rumour and uncertainty. It is known, however, that Lee made one trip to China in 1988 with his wife. What happened on that trip is obscure, but the circumstances of the visit were extremely odd.
Sylvia Lee was officially employed as a secretary at Los Alamos, but it was she, and not her highly qualified husband, who was mysteriously invited to China to give an academic paper on high-technology computer processing.
Whatever the connection, the US government now says, by the early 1990s reports were beginning to emerge from spy satellites and other sources that China had started to test small nuclear bombs, similar in concept and effect to those on which the US believed it possessed a closely-guarded monopoly.
In 1995 American experts from Los Alamos analysed the latest information on Chinese nuclear testing and concluded that they showed crucial similarities to the most advanced US miniature warhead, the W-88.
The discovery inevitably triggered a security inquiry. Working in true John Le Carre style, Notra Trulock, the US Energy Department's intelligence chief, drew up a "worst-case theory" in late 1995 based on his study of a top-secret 1988 Chinese document passed to the CIA, and on an analysis of nine Chinese nuclear tests.
Trulock concluded the Chinese had leapt 15 years in their nuclear programme, and there was only one explanation. The 1988 document specifically mentioned the W-88 and several of its design features. "They could not have done it without information from the US," Trulock told one briefing.
By early 1996 Trulock had narrowed his investigation to target five possible suspects. When a team of FBI investigators back-tracked through the travel and work records of the five suspects, one suspect "stuck out like a sore thumb", according to an official who spoke anonymously to the New York Times last weekend. That suspect was Wen Ho Lee.
In April 1996 a group of investigators decided to brief the White House about their suspicions. What happened between then and now, as the investigations continued and stalled, then stalled and resumed once more, is a matter of fierce dispute. With the allegations against Lee now at last in the public domain, all the agencies are desperately positioning themselves to push any blame on to their rivals in the event that they turn out to be true. One thing that is clear, however, is that Lee continued to work at Los Alamos right up to last weekend.
In December Lee took and failed a polygraph lie-detector test. Last Friday he was taken in for questioning by the FBI, but "stonewalled" the investigators at every turn, the US Energy Secretary, Bill Richardson said yesterday. Finally, on Monday, Richardson ordered the University of California to dismiss Lee from his job at Los Alamos. Officially, Lee was fired for failing to properly inform the authorities about his contacts with people from a sensitive country. Even now, though, Lee has not been charged with any crime, and the FBI acknowledged last week that it does not possess enough information to arrest him.
No one can say for certain whether Wen Ho Lee is really the latest in the long series of spies whose activities have ensured that US nuclear know-how has come regularly into the hands of its enemies over the past half-century. What is certain, however, is that the allegations have come at a time both helpful and embarrassing for the Clinton administration.
Both in Beijing and Washington, senior officials and observers suspect that the story has surfaced at this time in order to bolster President Clinton's plans for a controversial Star Wars theatre missile defence system.
Clinton's recent endorsement of this pet Republican project has been justified to the American public in the context of the threat from "rogue states", notably North Korea, which is alleged to be developing the capacity to launch a nuclear missile at Japan and, in theory, at the US itself.
THE LEE case has been seized on by Republicans for obvious reasons. Clinton has been repeatedly charged with allowing US high-tech companies to export technology to China with wholly insufficient safeguards. Earlier this month, in a surprise turnaround, the administration blocked the California-based Hughes Electronics Corporation from sending two $450 million satellites to China for launching.
In China too, there is an unresolved argument. Chinese policy towards co-operation with the US continues to veer, as it has for decades, between a vision of the two countries in partnership as the world's greatest two powers, and a deep suspicion that Washington's real aim is to deny Beijing the status to which it believes it is entitled. If the Clinton administration gives off contradictory signals about China, then it is not alone in doing that.
And if the nations themselves cannot decide where their self-interest lies, then who can say with certainty where the loyalties of a Taiwanese-born scientist working in the most secret site in the world will lie? Things were so much more straightforward in the Cold War.