US President George W Bush has announced a series of proposals to try to stop the spread of nuclear weapons.
Pakistan began its nuclear programme in the 1970s
His speech followed the admission by the father of the Pakistani bomb, Dr AQ Khan, that he had given nuclear secrets to other countries, believed to be Iran, North Korea and Libya.
BBC News Online world affairs correspondent Paul Reynolds has been following the trail of the black market bombs.
The story begins in the early 1970s. An ambitious young Pakistani metallurgist Abdul Qadeer Khan (known in the style of the sub-continent by his initials AQ) was working in the Netherlands for a Dutch company called Physics Dynamic Research Laboratory.
Origins at Urenco
FDO, as it was called, did research for a consortium called Urenco, set up by the British, Dutch and German governments to provide equipment to enrich uranium.
Whether Dr Khan had gone there in order to get information needed to build a nuclear bomb is not known. What is known is that when, in 1974, India exploded its first nuclear device, he was well placed to help his own country.
Dr Khan was pardoned, despite his dramatic revelations
Specifically, he was able to get blueprints for a centrifuge made by Urenco.
Centrifuges are metal tubes which spin uranium hexafluoride gas in order to separate out the uranium 235 which is needed to make a nuclear reaction.
In this way uranium can be enriched to the level required for a nuclear power station but also to the higher levels needed for a nuclear bomb. Dr Khan had the higher ambition.
According to Frits Veerman, a technical photographer who worked in the same office at FDO, Dr Khan kept blueprints in his house, where Mr Veerman sometimes went for tea and fried chicken. Later he wrote to Mr Veerman after he left the Netherlands in 1976 with the Dutch intelligence hard on his heels.
Dr Khan asked Mr Veerman to get more details, opening one letter with the words: "Dear Frits, very confidentially I request you to help us."
Khan back in Pakistan
Armed with his blueprints, Dr Khan then set up the AQ Khan Research Laboratories near the Pakistani capital Islamabad and began to build the bomb, often getting supplies and equipment from European companies. In those days, controls were lax and in any event much of the equipment was dual use so its ultimate purpose could be hidden. Dr Khan was remarkably successful.
At some stage, however, he ceased to be satisfied with confining his work to Pakistan.
Whether this was because he realised that he could sell his expertise elsewhere, or whether he saw himself as a kind of nuclear mastermind countering American hegemony, is not really known.
Nor is it known to what extent, if any, the Pakistani government knew about his extra curricula activities. He is believed to have helped North Korea, which supplied Pakistan with missiles. Such an exchange could hardly have taken place without government to government contacts.
Trail to Libya
What is known, in broad terms, is the trail which led from Dr Khan to Libya and it can probably be reckoned that a similar path led to Iran, though the Libyan connection was more sophisticated.
President Bush himself laid out some of the evidence.
A key figure was BSA Tahir, a Sri Lankan businessman living in Dubai whom Mr Bush called Dr Khan's "deputy and chief financial officer and money launderer".
Mr Tahir, said Mr Bush, had set up a front company SMB Computers, to help the operation.
Mr Tahir is said to have placed an order for centrifuge parts with a Malaysian company named by the CIA as Scomi Precision Engineering. The cover story was that the parts were for the oil and gas industry.
Scomi said the actual order was placed by a British company in Dubai called Gulf Technical Industries (GTI) in which Mr Tahir had a partner named Paul Griffin.
Mr Griffin has denied, in an interview with the Guardian newspaper, that he knew anything about the centrifuge order.
In any event, the order went through and the parts were delivered to Dubai.
It was after they were loaded on a German ship the BBC China, bound for Libya in the late summer of 2003, that western governments struck.
The ship was intercepted by the Germans and Italians and taken into an Italian port. There the "used machinery parts" listed as the cargo were found to be the centrifuges manufactured in Malaysia, probably to the designs of Dr Khan.
Libya had already opened talks with the US and UK about abandoning its work on weapons of mass destruction, so it is curious that Libya should also have continued with this shipment. There have been suggestions that Libya tipped the British and Americans off as a sign of good faith. If not, they were acting in bad faith.
Whatever the cause, the shipment was revealed.
What was even more worrying was that Libya showed the Americans and British a design for a nuclear warhead, which is believed to have originated with Dr Khan as well. Mr Bush said the Khan network even sold raw uranium at one stage, though to whom is not clear.
At the same time, Iran was having to admit that it, too, had acquired expertise from abroad.
The Iran operation pre-dated the one with Libya and was less sophisticated because it seems that Dr Khan simply gave the Iranians surplus equipment. He had over-ordered some parts for his own needs, so he had something to sell. He had also developed new centrifuges which meant that the old ones could go on the market.
One of them, which ended up in Iran, was apparently contaminated with enriched uranium. This was found by the United Nations nuclear agency, the International Atomic Energy Agency (IAEA). Iran was forced to admit either that it had processed the fuel itself or got it by accident. It chose the latter explanation, but that opened up the whole question of where it came from.
Iran simply said it got the parts through a third party. But technical analysis has detected the hand of Dr Khan in the designs.
Looking back, it was amazing that Dr Khan managed to carry on for so long. A former British envoy in Pakistan has said that he did challenge the Pakistani authorities about Dr Khan but was assured that all was in order.
The unravelling of his network will partly make up for the previous intelligence failure but the lapse is highly worrying for western governments. That is why President Bush has proposed the new measures.
Closing the loophole
In particular they would close a loophole which allowed Dr Khan to operate internationally.
Under the nuclear Non-Proliferation Treaty, which came into effect in 1970, countries without nuclear weapons are allowed to develop nuclear power and, crucially, are allowed to make their own fuel.
But enriching uranium beyond power station grade to weapons grade is no great technological feat and as long as you keep your activities a secret, you can get away with it.
The United States now wants to confine fuel enrichment to those countries which already have the capacity to do it.
In that way, everyone else would buy their fuel from recognised sources and there would be no other fuel enrichment going on.
It would not necessarily stop another Dr Khan but it would make such an undertaking much more difficult.