Actually, Ulysses S Grant clamped down very brutally on groups like the KKK during the Reconstruction Era, essentially continuing the civil war against the south in a sense. He ended up reducing their strength significantly and driving them underground.

Unfortunately, an idea cannot be destroyed with military might, it can only be fought by teaching critical thinking skills and sound information gathering and decision-making methods, so people don’t think so stupidly.

https://en.m.wikipedia.org/wiki/Enforcement_Act_of_1870

Note the line that says the law allows the use of the army to enforce it, that’s what Grant ended up doing.