WaPo has an excellent
timeline of Libby's role in the CIA leak case.
In February 2002 the CIA sent Wilson to Niger to investigate the report that Iraq tried to buy "yellowcake" uranium from Niger for nuclear weapons.
In March 2002 Wilson briefed the CIA on his findings. The agency sent a memo to the White House.
On Jan 28 2003 Bush gave his State of the Union address and included the famous 16 words
"The British government has learned that Saddam Hussein recently sought significant quantities of Uranium from Africa"
The memo was sent to the White House, so Bush must have been aware that this information in his speech was false.
Libby , along with the Cheney's White House Iraq Group then moved into action to protect the president when Joe Wilson brought out the fact that Bush had lied in the State of the Union address about the Niger Uranium.
The President thus is responsible for using knowingly false information to lead us war against Iraq and for covering it up when it became clear he lied.