Reconstruction was ended too early, in large part because of the Lincoln assassination, and the very questionable choice to appease the South by putting Andrew Johnson a heartbeat away from the presidency

We definitely should have been draconian in punishing the south, but our greatest mistake is civility politics. The leaders were all in the same social circles, and to this day that's a recurring problem that plagues most democracies. They didn't want real consequences for their buddies

So all in all, we didn't strip enough power from the leadership and stopped punishment way earlier than even the overly light-handed plan in place