Societies in all countries are split by major divisions or "fault lines" caused by differences in race, religion, ethnicity, wealth, class, or power. Some are plainly evident, whereas others are more concealed and can erupt with little warning.
In a world facing acute environmental, migration, and resource challenges that can only exacerbate differences, this is an essential guide to understanding a phenomenon that all countries must grapple with in the twenty-first century.
Jeffrey Herbst is the president of Colgate University, New York.
Terence McNamee's books include Century and Decade (Phaidon Press).
Greg Mills' most recent book is Why Africa is Poor (Penguin Books).