Members of the 30 September Movement attempted a coup against the Indonesian government that was crushed by the military under Suharto, leading to a mass anti-communist purge with more than 500,000 people killed over the following months.

The Thirtieth of September Movement was a self-proclaimed organization of Indonesian National Armed Forces members. In the early hours of 1 October 1965, they assassinated six Indonesian Army generals in an abortive coup d'état. Later that morning, the organization declared that it was in control of media and communication outlets and had taken President Sukarno under its protection. By the end of the day, the coup attempt had failed in Jakarta. Meanwhile, in central Java there was an attempt to take control over an army division and several cities. By the time this rebellion was put down, two more senior officers were dead.

Suharto was an Indonesian military general, politician and dictator who served as the country's second and longest-serving president from 1967 to 1998. Rising to prominence amid political turmoil and anti-communist purges in the mid-1960s, Suharto gradually sidelined Sukarno and formally assumed the presidency after MPRS force Sukarno, to resign. His three-decade rule, characterised as authoritarian and kleptocratic, was marked by widespread corruption, political repression, and human rights abuses. Suharto's regime ultimately collapsed in 1998 amid mass protests, violent unrest, and the fallout of the 1997 Asian financial crisis, leading to his resignation.

Large-scale killings and civil unrest primarily targeting members and supposed sympathizers of the Communist Party of Indonesia (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 million people were killed, with some estimates going as high 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.