Final answer:
The First Reconstruction involved harsher treatment of the Southern states driven by the Radical Republicans.
Step-by-step explanation:
The period of Reconstruction that involved harsher treatment of the Southern states driven by the Radical Republicans was the First Reconstruction. This period, which took place from 1865 to 1877, was characterized by the Radical Republicans' push for a more equal society and their efforts to dismantle the old southern order. They advocated for federal control over the process of reconstruction and sought to protect the rights of freedpeople.