In the late 1950s, the state of Israel decided that in order to maximize its future chances of survival against a demonstrated pervasive regional threat it needed to pursue the acquisition of nuclear weapons. With the assistance of France, under the auspices of a secret agreement with the French government, Israel began the process of building a working nuclear reactor in the Negev near the town of Dimona. Since Israel was explicitly not interested in joining the international regime which was eventually formalized in the Treaty on the Non-Proliferation of Nuclear Weapons, the reactor construction was kept as secret as possible. Components of the reactor shipped from France were mislabeled in order to pass French export controls and customs agencies rather than creating telltale paper trails. In addition to support and components for the reactor, France supplied the nascent Negev Nuclear Research Center with nuclear fuel for the reactor.
On June 5, 1967 a long series of incidents and policies within Israel and its neighbors culminated in Israel's pre-emptive attack on the Egyptian Air Force as the opening move of what would be called The Six Day War. Following this short conflict and associated decisive Israeli victory, Europe and the United States were faced with an Israel which, while not only clearly the technical aggressor, was in possession of conquered territory that it announced it had no intention of returning or allowing UN intervention in. As one consequence, the French government withdrew support for the Israeli nuclear program.
This wasn't crippling from a technical point of view, as the reactor was complete by this time. However, it meant an interruption in reliable access to nuclear fuel for the Dimona reactor. Israel began to cast about for a new source of fuel. She was hampered by the strict controls on nuclear materials dictated by the IAEA and the regional European atomic control organization EURATOM.
Unable to purchase fuel openly due to the "policy of ambiguity" regarding the existence of the Dimona reactor and associated weapons research, and due to the negative repercussions of the Six-Day War, Israel decided to acquire fuel by whatever means possible. The result was a foreign covert operation carried out by the Mossad and Lekem, a secret Israeli intelligence agency concentrating on scientific and technical espionage. With no prospect of purchasing the requisite fuel, Israel set out to steal it. The story of what happened next is famous and widely told in books ranging from acknowledged fiction to research histories.
The bare bones of the story is that the Israelis managed, through front companies and deception, to purchase 200 tons of yellowcake - a concentrate of uranium ore - in Europe ostensibly to ship from Antwerp to Genoa for chemical processing. The 'front' was that the material would be processed at a chemical plant near Milan, then shipped back to Antwerp for use as chemical precursor. In reality, Operation Plumbat was a complex dance and shell game during which the yellowcake, sealed into 200 liter oil drums labeled simply Plumbat (from the Latin Plumbum or lead), were loaded on board a ship named the Scheersberg A, which sailed out of Antwerp on November 16, 1968.
It never arrived in Genoa. From the point of view of the world, the Scheersberg A simply vanished from the earth for several days. A week or so later, it 'turned up' in Palermo, Sicily, empty, with the most recent pages of its logs missing and no crew. There was no sign of the yellowcake. So neatly was this done that it took several weeks for EURATOM to even realize that the 200 tons of yellowcake had been on the ship and were now missing.
Eventually, a few years later, the Mossad was tied to the Scheersberg A in a publicly verifiable manner, and what had happened (although not precisely how) became clear. Israel, for its part, never acknowledged any of this, but Operation Plumbat had successfully netted Israel enough nuclear fuel to construct between 20 and 200 nuclear weapons (depending on type and size).
Ken Follett wrote a well-known novel titled Triple which told a thinly-fictionalized version of the story of Operation Plumbat. There have been numerous investigative journalism efforts and histories which have sought to detail the events of the operation as well. 1978's The Plumbat Affair was an investigative rather than fictionalized account, for example. (It's available online, but I've been warned by some that the site it is hosted at - http://saba.fateback.com/plumbat/plumbatindex.htm - may attempt to install malware on your browser or computer. I didn't see anything, but I'm on a Mac and thus probably wouldn't be targeted). The full story is fascinating and complex, far beyond the ability of this simple node to convey. The espionage is made all the more thrilling by the fact that it is a true story.