This part of the story began in 2002, when Joe Wilson, a former US ambassador, was sent to Africa to investigate a spies' fairytale about Saddam Hussein seeking uranium from Niger. Despite his conclusion that the story was baseless, George Bush used it in a speech setting out the case for war two months before the invasion. When, after the invasion, Mr Wilson took issue with the President in this newspaper and the New York Times, the administration retaliated. The President's men were so eager to discredit Mr Wilson that they leaked the fact that his wife was an undercover CIA agent. In Washington's internal turf wars, the CIA had long urged the Defense Department hawks to be cautious about their search for pretexts. The implication was that of course Mr Wilson would be against the war because he was married to a member of the sceptical fifth column.
It was an extraordinary and reckless leak. The phrase "putting lives at risk" is overused in the world of espionage, but in this case it is no more than a factual description. Mr Wilson's wife, Valerie Plame, is safe enough, but who knows what has happened to her contacts abroad.
If Mr Libby is guilty as charged, he has committed a serious offence. This is no mere inside-the-Beltway game. In contrast to the vexatious and politically motivated persecutions of Bill Clinton, this is an ex- ample of the American system with its independent prosecutor working well: it is holding an arrogant administration to account. And, as ever, what got Mr Libby into trouble was not only the original offence but the cover-up: the attempt to deny wrong-doing and to obstruct the investigation.
There are implications for Tony Blair, too. Because the US political structure is more transparent than ours, the Bush administration has been forced to admit that the nukes-from-Niger story was fictional - leaving Mr Blair isolated as the one leader who pretends to have "other sources" for it.
The real danger to the Bush-Blair alliance, however, lies in the further damage this episode will do to its credibility on Iraq. In recent months the American public has been less and less willing to believe that the situation on the ground can ever improve while US troops are deployed there, despite Mr Bush's reassurances. The more the original case for the invasion is called into question, the more the President's credibility will be eroded. Both in Iraq, where the American and British death toll continues to rise steadily, and in the White House, where one head has metaphorically rolled with more likely to follow, the Bush-Blair policy on Iraq lies increasingly in ruins.