The prosecution of Lewis “Scooter” Libby is set to become a political show trial to be relished by opponents of the Iraq war. The roots of the scandal that led to his being charged with perjury lie in how the White House justified the invasion, and the subsequent spin operation that it mounted after the failure to find weapons of mass destruction.

In one key respect the six- week trial, which begins on Tuesday, is unprecedented: the relentlessly secretive Mr Cheney will enter the witness box as part of Mr Libby’s defence. It makes him the first sitting vice-president to testify in a criminal prosecution. He is expected to be cross-examined on his efforts to rebut critics of the war aggressively as it began to turn sour in 2003, an experience he will find detestable.

Mr Libby denies lying to the FBI and a grand jury about who gave the press the name of Valerie Plame, a former covert CIA agent with stunning good looks. He faces up to 30 years in jail if convicted.

No Bush Administration official, including Mr Libby, was charged with leaking Ms Plame’s identity. It became clear last year that the accusation that triggered the special prosecutor’s investigation — that the White House knew that Ms Plame was an undercover operative and leaked her identity deliberately to discredit her husband, a war critic — was unfounded. Mr Libby is now accused of a covering up a non-existent crime that did not need to be hidden.

The genesis of the Plame scandal lies in a CIA-sponsored trip that Joseph Wilson, Ms Plame’s husband and a former ambassador, made to Niger in 2002 to investigate reports that Iraq had tried to buy uranium there. In July 2003 he accused the Bush Administration of ignoring his report that there was no Iraq-Niger link, and of twisting prewar intelligence.

A week later Robert Novak, a conservative columnist, wrote that Mr Wilson had been sent to Niger by Ms Plame, the first time that her name had been made public. The report caused a political storm after Mr Wilson claimed that his wife was a covert agent and that her name had been leaked as retribution for his criticism of the war. It is a crime knowingly to reveal the identity of a covert official.

Soon after Mr Novak’s column, several other journalists wrote about Ms Plame. They told Patrick Fitzgerald, the investigating prosecutor, that they learnt about her from Mr Libby and Karl Rove, Mr Bush’s chief adviser. Mr Libby is charged with lying to investigators because he claimed that he first learnt about Ms Plame from the reporters. One journalist, Judith Miller, was jailed for 85 days after refusing to reveal that her source was Mr Libby.

Claims of a White House operation to destroy the Wilsons were all but demolished after it emerged recently that Mr Novak’s source was Richard Armitage, Colin Powell’s former deputy at the State Department. He was a war critic and mentioned the Plame-Wilson connection as “idle gossip”.

Mr Fitzgerald will try to show that Mr Cheney was obsessed with pushing back against Mr Wilson and that it was a “high priority” for his chief of staff — and that Mr Libby tried to obscure that fact.

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