There’s a paragraph in George Tenet’s book, At the center of the storm: My years at the CIA, which confirms that the US planned to invade Afghanistan before 11 September 2001. It goes like this:
“At eight-thirty that evening [11 September 2001], speaking from the Oval Office, the president addressed the nation in terms both stirring and deeply earnest, including the first enunciation of what became known as the Bush Doctrine. “I’ve directed the full resources of our intelligence and law enforcement communities to find those responsible and bring them to justice”, he told a global audience of some eighty million people. “We will make no distinction between the terrorists that committed these acts and those who harbor them.” For us at CIA, the new doctrine meant that restraints were finally off. We already had on our shelves the game plan for going after al Qa’ida and its protectors, the Taliban, in Afghanistan .Now we could begin to implement it. Amid the sorrow of the day, we realised that we were finally going to be given the authorisation and the resources to do the job we knew had to be done.” (My italics for emphasis.)
As if we didn’t already know! This, however, is primary evidence.
 George Tenet, At the center of the storm: My years at the CIA. (New York: Harper-Collins, 2007.) p. 170-171.