The timeline.

An excerpt:

Given the chronology and Mr. Armitage’s remarks, it seems quite obvious Mr. Wilson outed his wife when he spoke to the Senate Democratic Policy Committee and then to the subsequent reporters at the Times, the Post and elsewhere, when he was hawking his story about his trip to Niger.
Wilson’s motivation for bringing up his wife would have been exactly as Armitage suggested to Woodward. Wilson told the panelists and reporters about Plame’s work at the CIA to give his radically new and dangerous story more credibility.
It’s highly probable Wilson used his wife’s position as a WMD analyst at the CIA to bolster his outrageous (and we now know fallacious) claims against a then popular President in a time of war.