The balance of the probabilities is that Scooter Libby perjured himself and obstructed justice because he knew things about Cheney that were deeply embarrassing and discrediting, and because he believed that Patrick Fitzgerald was a special prosector like Kenneth Starr--one who believed that the core of his job was to leak as many grand jury secrets as he could as long as they were discreditable to the president's party. Thus Libby thought that perjury was the better part of valor given that he needed to protect his boss from the discreditable and embarrassing possibility that people would learn how he behaved.
There is also the possibility that Scooter Libby perjured himself because for him to testify truthfully would have led to the impeachment and removal from office of Cheney and Bush.
Now we will never know which.
: Just another obstruction of justice http://commentisfree.guardian.co.uk/marcy_wheeler/2007/07/libby_sentence_again.html: On June 9, 2003, just one day after his national security advisor, Condoleezza Rice, got beaten up on the Sunday shows for claiming no one in the administration knew that the Niger intelligence was bunk, George Bush expressed concern about the allegations. Scooter Libby passed on that concern to vice president Cheney. Bush's concern set off a chain of events that ended up in the outing of a CIA spy, Valerie Plame, and the indictment and conviction of Scooter Libby.
Yesterday, George Bush attempted to prevent that chain of events from continuing any further. He commuted Scooter Libby's 30-month sentence. Rather than serving time in jail, Libby will remain free, with a fine and probation as the only remaining punishments for lying and obstructing a criminal investigation. But the real effect of Bush's actions is to prevent Libby from revealing the truth about Bush's - and vice president Cheney's - own actions in the leak. By commuting Libby's sentence, Bush protected himself and his vice president from potential criminal exposure for their actions in the CIA Leak. As such, Libby's commutation is nothing short of another obstruction of justice.
Cheney's involvement in the CIA leak case is central. He personally undertook research on Joe Wilson and his trip; while doing that research, Cheney learned that Wilson's wife worked in the counter-proliferation division of the CIA, the part of the clandestine services that fights the spread of weapons of mass destruction. Cheney then passed on the news of Plame's CIA identity to Libby.
When, on July 6, 2003, Joe Wilson publicly accused the administration of fixing the case for war in an op-ed in the New York Times, Cheney cut out the op-ed and wrote his notes on it. "Have they done this sort of thing? Send an ambassador to answer a question? Do we ordinarily send people out pro bono to work for us?" After writing those questions about the circumstances of the trip, he wrote one more question, making an accusation that would show up later in leaks to journalists. "Or did his wife send him on a junket?" When Joe Wilson criticised the administration publicly, Cheney's response was to insinuate that Wilson's wife - whom Cheney knew worked in the clandestine side of the CIA - was somehow doing something improper.
The next day or the day after, Cheney ordered Scooter Libby to leak something to the New York Times' Judith Miller. Libby hesitated. He couldn't leak it, Libby said, because it was classified. But Cheney reassured Libby that President Bush had unilaterally declassified the material. Libby was still worried, so he asked Cheney's counsel, David Addington, whether the president could just unilaterally declassify something. When Addington assured him that the president could, Libby seemed satisfied.
Shortly thereafter, Scooter Libby leaked to Miller Valerie Plame's identity and the contents of the CIA report on Wilson's trip to Niger. Libby would later say that Cheney had declassified the NIE, and not Plame's identity or the trip report. But there's no paperwork to support this claim, just the word of a convicted perjurer. Furthermore, Libby testified that Cheney had ordered him to leak this material exclusively - and Libby had already leaked the NIE to two other reporters. The two pieces of information that Libby leaked to Miller exclusively, after Cheney ordered Libby to leak information to her, were Plame's identity and the CIA report on Wilson's trip.
The following Monday, July 14, when a Robert Novak article published Plame's covert identity, Cheney and Libby already knew it would appear. The first thing they did that morning was ask their CIA briefer if he had read it - they told him that it was not his problem. As if they already knew it was somebody's problem.
Later that fall, as the scandal erupted, Libby asked Cheney to ensure that the White House spokesperson publicly exonerated him, as he had earlier done for Karl Rove. To make sure this happened, the Vice President started to write a note, "Not going to protect one staffer [meaning Rove] and sacrifice the guy the President..." But then Cheney stopped. He crossed out the first four letters of the word, "President" and finished the sentence: "...that was asked to stick his neck in the meat grinder because of the incompetence of others." He came close to writing that President Bush had asked Libby to take the lead on responding to Wilson. And then, according to some trial testimony, he got Bush to make sure Libby got his public exoneration, a public claim that Libby had nothing to do with the leak of Plame's identity.
Then, finally, before Scooter Libby told the FBI his now-discredited story about learning Plame's identity anew from Tim Russert, Libby went and told the vice president what he was going to tell them. The only person, apparently, that Libby shared his lies with was the one guy - the vice president - who knew they weren't true.
There are many unanswered questions about the roles of the president, the vice president, and Libby in the leak of Valerie Plame's identity. Did Bush really ask Libby to take the lead on all this? Did the president declassify Plame's identity so Libby could leak it to the press? Did Cheney learn - and tell Libby - that Plame was covert? Those questions all point squarely at Bush and Cheney personally. But because of Bush's personal intervention, he has made sure that Scooter Libby won't be answering those questions anytime soon.