Large-scale killings and civil unrest primarily targeting members of the Communist Party (PKI) were carried out in Indonesia from 1965 to 1966. Other affected groups included alleged communist sympathisers, Gerwani women, trade unionists, ethnic Javanese Abangan, ethnic Chinese, atheists, so-called "unbelievers", and alleged leftists in general. According to the most widely published estimates at least 500,000 to 1.2 million people were killed, with some estimates going as high as two to three million. The atrocities, sometimes described as a genocide or 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 controversial 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 upheavals led to the fall of President Sukarno and the commencement of Suharto's three-decade authoritarian presidency.
The abortive coup attempt released pent-up communal hatreds in Indonesia; these were fanned by the Indonesian Army, which quickly blamed 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 United States, 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 the neighbouring Federation of Malaya, a Commonwealth federation of former British colonies.
Communists were purged from political, social, and military life, and the PKI itself was disbanded and banned.