The nonlinear dynamical process of self-organized criticality provides a new ‘theory of history’ that explains a number of unresolved anomalies: Why are the really big events in human history usually unpredictable? Why is it impossible to anticipate sudden political, economic, and social changes? Why do distributions of historical data almost always contain a few extreme events that seem to have had a different cause from all the rest? Why do so many of our ‘lessons of history’ fail to predict important future events? As people, organizations, and nations become increasingly sensitive to each other's behavior, trivial occurrences sometimes propagate into sudden changes. Such events are unpredictable because in the self-organized criticality environment that characterizes human history, the magnitude of a cause often is unrelated to the magnitude of its effect.