What makes difficult history difficult?

All modern nation-states have periods of difficult history that teachers fail to address or address inadequately. The authors present a framework for defining difficult histories and understanding what makes them difficult. These events 1) are central to a nation's history, 2) contradict accepted histories or values, 3) connect with present problems, 4) involve violence enacted by the state or large groups of citizens, and 5) create disequilibria that require changes to historical understandings that may carry a personal or social cost.