Final answer:
The largest white supremacist group during Reconstruction was the Ku Klux Klan (KKK), which used terror to maintain white supremacy and oppose Reconstruction policies.
Step-by-step explanation:
The largest white supremacist group in the South during Reconstruction was the Ku Klux Klan (KKK). Founded in 1866 by Confederate veterans, the KKK became an organization synonymous with terror and violence against African Americans and their allies. Its purpose was to resist the policies of Reconstruction and to maintain white supremacy through intimidation and violence. Variants of Klan-like groups spread throughout the South, as white supremacists aimed to reverse the advances made by African Americans and to restore antebellum social structures.