By mid-1966, about half a million adherents of the Indonesian Communist Party (PKI) were dead, many more were arbitrarily imprisoned and even more lost civil rights. The biggest communist party outside the communist bloc disappeared almost overnight, as did its affiliated social organisations. It was the worst political violence since Indonesia´s 1945-1949 war of national liberation. How could this happen? The chapter first dismisses two once-popular analytical approaches. Neither behaviourist depictions of rampaging anti-communist crowds, nor statist images of a military conducting pogroms on its own are adequate to the known facts. It then develops a contentious politics approach with multiple collective actors. The cold war looms large; the economy is politicised; institutions are weak, factionalised, and deeply embedded in various social formations. Contention escalates from September 1963, as President Sukarno and the PKI pivot from the gradualist Soviet Union to a militant People´s Republic of China. An emerging legitimation crisis pits a social justice discourse popular among lower classes against a growing middle class religious, law-and-order discourse. When the PKI leadership makes a false move on 1 October 1965, the military mobilises its allies to strike back, with genocidal results.