This essay addresses the question, “Why did one-drop triumph at this time and not before nor after?” It suggests a hypothesis in six topics. The One-Drop Rule Punished Entire Families, not Just Individuals shows that, although the court cases dealt with individuals, entire families were actually punished. The One-Drop Rule was Known to be Irrational presents evidence that one-drop trials were not searches for either factual accuracy or for moral justice. The One-Drop Rule was Wielded Against Whites, not Against Blacks shows that the victims were White. To be sure, some victims may actually have had recent African ancestry, as do one-third of White Americans. But if this made them Black, then it means that one-third of all White Americans were also Black and the question remains—why pick these out? Why Did it Happen surveys the literature for the causes of the Jim Crow wave of terror itself. The One-Drop Rule Kept White Families in Line presents this study’s hypothesis that one-drop was an instance of a well-studied phenomenon of group dynamics involving ideological self-preservation. Other Voices offers an alternative explanation.