Leading article: Nods, winks and a lesson for the media worldwide

As legal theatre, the drama being played out in Courtroom 16 of the federal district courthouse in downtown Washington DC this week takes some beating. Judith Miller, the fallen star reporter of The New York Times, has been in the witness box testifying against Lewis "Scooter" Libby, the former chief of staff for Dick Cheney, probably the most powerful Vice-President in American history.

In 2005, Ms Miller spent almost three months in jail for contempt of court for refusing to identify Mr Libby as the source of confidential information related to the trumped-up case against Saddam Hussein that the Bush administration, led by Mr Cheney, used to justify the disastrous war in Iraq. Mr Libby is on trial on charges of lying to a grand jury over his involvement in the leak of the identity of the CIA agent Valerie Plame, whose husband, a former ambassador, had publicly debunked sensationalist claims by the White House that Saddam had sought uranium in Africa - the basis of allegations that the dictator was building a nuclear weapon. Ms Miller, the reporter Mr Libby had sought to manipulate, is now one of his prime accusers.

But if this case is haunted by the WMD fiasco, it is not about the WMD fiasco. To be sure, the trial comes at an awful moment for the administration. Iraq is collapsing into civil war, President Bush's Republican Party is nearing mutiny, and he himself is little more loved than Richard Nixon at the height of Watergate. The last thing the White House would have wished for is a procession of top officials through a public courtroom, once more reminding the public of the specious grounds on which the invasion was launched. Now, precisely that is happening. Indeed, Mr Cheney is himself shortly due to testify in Mr Libby's defence.

But the war itself is not on trial. The events in question took place in summer 2003, three months after Saddam was toppled. A court has not pronounced judgment on the invasion and, short of an impeachment of Mr Bush, probably never will. But public opinion, as expressed by the President's abysmal approval ratings, most certainly has. And so - barring Mr Cheney and a few diehard neo-conservatives - have experts and historians.

The Libby affair is hugely important nonetheless. It has turned a rare and merciless spotlight on the system of nods, winks and leaks that operates between senior officials and the media that cover them. The former, like Mr Libby, fight their internecine battles (in this instance he was taking aim at the CIA, which he believed was backpedalling from the famous "slam dunk" WMD intelligence the agency's director at the time, George Tenet, boasted of before the war). Reporters like Ms Miller are vouchsafed nuggets of information on the understanding they will not reveal where they came from. The system has perhaps reached perfection in Washington. But similar things go on in London, Moscow, or anywhere else that politics is practised. And, as the Libby trial indicates, the system has its pitfalls.

Ms Miller acted bravely in choosing to go to prison rather than reveal her source - and remained there until Mr Libby explicitly allowed her to name him. The case thus dispels any notion that US journalists have de facto legal immunity over the protection of sources. In future whistleblowers in government may be less inclined to provide information truly in the public interest - unlike the nonsense peddled by unspecified "senior US officials" over Saddam's non-existent weapons. But Ms Miller is villain as well as heroine. She may never have written a word about Ms Plame. But the erroneous stories she had written earlier about WMD underline a lesson for journalists everywhere. If unnameable sources dangle a scoop in front of you, make sure it's true.