In February 2002, Joseph Wilson, a former US ambassador was sent to Niger in West Africa to investigate intelligence that Iraq had agreed the purchase of uranium yellowcake (refined uranium ore) there.
Since Iraq had no civilian nuclear programme, the supposition was that it was seeking a source of fuel for a military one.
The British government certainly believed the Niger report, at least to the extent that Iraq had made approaches to Niger - and strangely enough after all that has happened, still does, even though the CIA does not.
London used it in its intelligence dossier against Iraq. Mr Bush himself referred to it in his State of the Union address in January 2003.
But Mr Wilson said he found no evidence of any sale. In an article in the New York Times after the war he quoted news reports that documents purporting to show a sale were probably forged. Nor did he report any clear evidence that Iraq had approached Niger for a sale.
Nothing much might have happened had Mr Wilson not written that article. In it he stated: "A legitimate argument can be made that we went to war under false pretences."
Eight days later, a veteran Washington columnist Robert Novak, wrote that Mr Wilson's wife Valerie Plame (her unmarried name) was a CIA agent whose job was to analyse the spread of nuclear weapons.
Novak said that "two senior administration officials" had told him that Valerie Plame herself had suggested the Niger trip.
The implication was that Ms Plame had put work and prestige her husband's way and, according to a Democratic Congressman, Henry Waxman, was part of a "smear campaign" against Mr Wilson.
Intriguingly the indictment says that Mr Novak was told about Valerie Plame by someone at the White House simply described as Official A. This official is now said in news reports to be Karl Rove.
The knowing disclosure of an undercover CIA agent is illegal, so the hunt started for the source of the leak.