A new theory of ethnic cleansing based on the most terrible cases (colonial genocides, Armenia, the Nazi Holocaust, Cambodia, Yugoslavia, Rwanda) and cases of lesser violence (early modern Europe, contemporary India, and Indonesia). Murderous cleansing is modern, 'the dark side of democracy'. It results where the demos (democracy) is confused with the ethnos (the ethnic group). Danger arises where two rival ethno-national movements each claims 'its own' state over the same territory. Conflict escalates where either the weaker side fights because of aid from outside, or the stronger side believes it can deploy sudden, overwhelming force. Escalation is not simply the work of 'evil elites' or 'primitive peoples'. It results from complex interactions between leaders, militants, and 'core constituencies' of ethno-nationalism. Understanding this complex process helps us devise policies to avoid ethnic cleansing in the future.