Without Nathan Forrest I dont think the KKK would have got off the ground. They were weak in numbers and powers up to 1900 IMO. I think w/o the Nativist - anti immigrant movement between 1900 and 1920 I dont think the Klan would have gone anywhere. In order to succeed, it seems like history bares out that in order to succeed you have to have someone to hate
I might add that without Parson Brownlow's extreme views and actions against the former Confederates, that the thing wouldn't have gotten off the first floor because decent men like Forrest would not have seen a need to join it. His farewell message to his troops shows he had every intention of following the law of the land and being a loyal citizen, encouraging his men to do the same. They expected to be treated as loyal citizens after they'd done all required of them to regain that status. Didn't work out that way by a long shot.