Valerie Plame’s husband, Joseph C. Wilson, investigated
uranium sales in Africa for the CIA. In 2002, he presented evidence in the New
York Times that statements George W. Bush made about Iraq importing uranium
were untrue. In 2003, columnist Robert Novak revealed that Valerie Plame
secretly worked for the CIA. Such employment is classified information, which
is illegal to reveal. Such revelations can be deadly for those who work for the
agency, and dangerous for the United States. A criminal investigation followed.
Scooter Libby was found guilty of lying to investigators, but his sentence was
commuted by George W. Bush.
Republicans were hugely critical of Joseph Wilson's credibility and qualifications after his public discussions of his CIA sponsored trip to Niger. He also came under fire for his criticisms of the Iraq War. Looking back, it seems that some of those may have been justified. Valerie Plame seems to have been an elite level spy, but her career is shot after being outed.
Joseph C. Wison's wife Valerie Plame was outed as a CIA agent in retaliation for his stance against the Iraq War. This is the most reasonable explanation that we know of because there was no reason why this would happen any other way. Even though President Bush denied such a rumor.
Since Joseph C. Wilson was against the Iraq War, his wife was outed as a CIA Agent. Such an explanation is completely logical for Valerie Plame's outing. There's no reason that her identity should or would have been revealed otherwise. Wilson's stance against the war was the main contributing factor.
This sequence of events, though not definitive, is still entirely plausible. Bush claims he knew nothing about Plame before the outing, and there is no reason to deny this. However, a number of people in the Bush staff could easily have leaked the information, something that Bush specifically condemned in his autobiography.