To spy on a U.S. citizen in the middle of an election, one who had just been working with one of the two major presidential campaigns, the FBI touted a gossipy, unverified, unreliable rag that it had no reason to believe and every reason to distrust, but it hid all of that from the FISA court, which it knew needed to believe that the Steele Dossier was something it was not if it were to give the FBI the spying authorization it wanted.
