Abraham Lincoln's one of my ideas because it was his death that pretty much put the breaks on the original plan on reconstruction, which involved punishing the south for seceeding and possibly jailing major characters involved in the Confederacy. Also, there was supposed to be protection and other details for now freed African Amercians that would have better ensured that the south wouldn't take advantage and continue power abuse against African Americans.
After Lincoln was assassinated, Andrew Johnson, Lincoln's vice president, let the former confederacy off with a slap on the risk, allowed laws that took advantage and straight up abused the rights of African Americans to pass, which led to even harsher laws being passed, and possibly pushed African American civil rights to not be fully realized until a century later.