From October of 1965 to March of 1966, a series of large-scale killings and civil unrest primarily targeting members and supposed sympathizers of the Communist Party of Indonesia (PKI) were carried out in Indonesia. Other affected groups included alleged communist sympathisers, Gerwani women, trade unionists, ethnic Javanese Abangan, ethnic Chinese, atheists, accused "kafir" (unbelievers in Islam), and alleged leftists in general. According to the most widely published estimates, at least 500,000 to 1 million people were killed. Some of the higher estimates reach figures as much as 2 to 3 million. The atrocities, sometimes described as a genocide or a politicide, were instigated by the Indonesian Army under Suharto. Research and declassified documents demonstrate the Indonesian authorities received support from foreign countries such as the United States and the United Kingdom.
The killings began as an anti-communist purge following a so-called attempted coup d'état by the 30 September Movement. It was a pivotal event in the transition to the "New Order" and the elimination of PKI as a political force, with impacts on the global Cold War. The Army, after banning all independent news source from issuing for a week, almost at once pinned the abortive coup attempt to the PKI. Additionally, the intelligence agencies of the United States, United Kingdom and Australia engaged in black propaganda campaigns against Indonesian communists. During the Cold War, the U.S., its government, and its Western allies had the goal of halting the spread of communism and bringing countries into the sphere of Western Bloc influence. Britain had additional reasons for seeking Sukarno's removal, as his government was involved in an undeclared war with neighbouring Malaysia, a Commonwealth federation of former British colonies.