I read this in Frederick Forsyth's The Day of the Jackal.
"Like most fanatics, Rodin could blind himself to facts with sheer belief." That's illuminating, given the situation we're now in. We're dealing with people who believe things that aren't true. dump set them up for that with his constant lies and attacks on those who told the truth. His "Big Lie" is much, much bigger than our current understanding of it. His biggest lie is that truth is false.
So how do we deal with the believers of the lies? I can think of at least two ways. We can emphasize, constantly, that their leader betrayed them. Take away the focus of their belief, and they become unmoored. Then we can blunt their ability to hurt us.
Infiltrate them, gather damning evidence, arrest and indict them, put them in prison. Breaking up and scattering their organizations and bases of power will not give us total protection against random terrorist attacks, but it will make them more diffuse. These tactics have been used against terrorists in many first-world countries. We must employ them here.