A special prosecutor is appointed to investigate a crime which never happened. Some inconsistencies in Lewis Libby's testimony lead to an indictment for perjury. Perjury is a serious crime. Everyone agrees this when the person under suspicion is a Republican. But the prosecution still has a mountain to climb. They have to prove that Libby deliberately lied. The precise details of a conversation may be difficult to recall months after the event. It is not obvious that this is the blatant and deliberate lie of "I did not have sexual relations with that woman". We can presume beyond reasonable doubt that Bill Clinton was not suffering a memory lapse. This is far from obvious in Libby's case.
As for the original investigation, the prosecutor found no evidence that a crime had even been committed. This should surpise no-one. Neither Rove nor Libby advised the press that Valerie Plame was covert. It seems unlikely that they even knew. Only one person has demonstrably lied in this kerfuffle - Joe Wilson. And the person who first announced to the world that Plame had covert status? That would be Joe Wilson too.
