Mr Ames's position, as head of the Soviet section of CIA counter- intelligence, is roughly comparable to that of Kim Philby, the head of the Soviet division of MI6, who, along with the British diplomats, Guy Burgess and Donald Maclean, was at the core of the 'Cambridge ring' of Soviet agents exposed in the 1950s and 1960s.
For the next two decades, scandal after scandal plagued British intelligence: some were quite separate cases - such as those of George Blake and John Vassall - but most involved further ramifications of the original Maclean, Burgess and Philby affair, which grew to embrace a 'Fourth Man' and then a 'Fifth Man'.
The United States was comparatively free of lurid espionage scandals apart from the still controversial case of Ethel and Julius Rosenberg who were executed in 1953 for passing atomic secrets to the then Soviet Union. None the less, fearful that the British blight had spread to the CIA, James Angleton - the agency's legendary former counter- intelligence chief - was convinced that a high-level Soviet 'mole' was active in its ranks.
But no mole was unearthed before Angleton was dismissed in 1974, and, unless his alleged career as a traitor started long before the date of 1985 contained in the Justice Department indictment, Mr Ames is unlikely to have been that man. The four CIA officers subsequently discovered to have been spies for the KGB are also unlikely to have been Angleton's mole.
The first of the four was Edwin Gibbons Moore, jailed in 1977 for 15 years for passing classified documents to the Russians. The following year, another CIA employee, William Kampelis was sentenced to 40 years in jail for handing over details of a secret US satellite. In 1980, David Barnett, an agency official working in Indonesia, was sent to prison for spying and taking dollars 92,000 ( pounds 62,000) from the KGB.
The most harmful Soviet agent uncovered within the CIA, and the only one to have escaped, was Edward Lee Howard. He was paid at least dollars 150,000 by the Russians for information which largely destroyed the CIA's operations in Moscow. Howard slipped out of the US in 1985 and turned up in Moscow a year later.
The sum of money alone which Mr Ames and his wife, Maria, are alleged to have received - at least dollars 1.5m - would be evidence of how precious was the information they are said to have passed on. It would almost certainly have included, as well as intimate details of the CIA's operations against the former Soviet Union and Russia, protection of Moscow's 'assets' in the US. Experts say the affair could be one of the most damaging ever to the US.
Apart from the four traitors from the CIA, Soviet agents have been discovered in the State Department as well as in the navy - some of whose most important secrets were handed over by the Walker father- and-son spy ring in the early 1980s.
But whereas British spies, at least those of the Philby vintage, appear to have been motivated by a misguided faith in Communism, the Americans tended to become traitors for money. The unprecedented sum alleged to have been paid this time suggests Mr Ames and his wife were no different.
In a June 1992 document, electronically copied by FBI agents and recorded in yesterday's indictment, Mr Ames asks his Soviet controller for cash: 'My most immediate need is money . . . Now I'm faced with the need to cash in investments to meet current needs . . . I've had to sell a CD in Zurich and some stock here to make up the gap. Therefore I will need as much cash delivered in Pipe (a 'dead drop' arrangement) as you think can be accommodated. It seems to me it could accommodate up to dollars 100,000.'
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