First of all, let's analyze the first point from the American society in the past. We all know that there has been serious racial discrimination in the United States since ancient times. The Klan we are going to talk about today is a political party with the slogan of white supremacy in American society. I think they caused serious racial discrimination in America. Then we can infer that since the Klan is a political party that protects the interests of white people, I believe that white Americans will definitely support them, after all, for their own interests. So this has led to the fact that the United States has always been white supremacy, which means that the Klan can rise and fall in American society.
Then the second point is analyzed from the perspective of white Americans. We all know that the United States is a multi-ethnic country, but as far as I know, the United States is still dominated by whites, which means that if the Klan defends the interests of whites, they will get the support of the majority of the people in this country. We can imagine that if a political party has many people's support in this country, its power will not be small. In addition, I think many white Americans will make suggestions for this party and even join it directly. So from this perspective, the rise and fall of the Klan in American society is mainly due to the support of white Americans.
Of course, the above is just my own guess. If you have other different opinions, you can add them! If you like my opinion, you can also give me a compliment!