Short answers, pulled from the story.
The Ku Klux Klan was founded on the 24th of December 1865 in Pulaski, Tennessee. Six former Confederate officers gathered in a tailor shop to create the organization that would become a feared terrorist group.
William Joseph Simmons founded the second Ku Klux Klan in 1915 atop Stone Mountain, Georgia. This iteration emerged as a nationwide movement inspired by the film The Birth of a Nation and adopted symbols like the burning cross.
The second Ku Klux Klan reached its peak membership in the mid-1920s with estimates ranging from three to eight million people. This made it one of the largest organizations in the United States at that time.
The second Ku Klux Klan declined in the late 1920s following scandals and the murder of Madge Oberholtzer by Grand Dragon D. C. Stephenson in 1925. These events shattered the organization's image and led to a dramatic collapse of its power.
The third Ku Klux Klan employed murder and bombings to pursue its goals, including the assassination of civil rights workers in Mississippi in 1964 and the bombing of the 16th Street Baptist Church in Birmingham in 1963. The group also engaged in conspiracy to commit robbery and bombing of natural gas processing plants.