The Communist Party of Indonesia (Indonesian: Partai Komunis Indonesia, PKI) was a political party in Indonesia. With growing popular support and a membership of about 3 million by 1965, the PKI was the strongest communist party outside the Soviet Union and China. The party had a firm base in various mass organizations, estimates claim that the total membership of the party and its frontal organizations might have at its peak organized a fifth of the Indonesian population. In March 1962 PKI joined the government. PKI leaders Aidit and Njoto were named advisory ministers.
Following the military coup in 1965, between 300,000 and one million Indonesians were killed in the mass killings that followed as the new regime cracked down on PKI. A CIA study of the events in Indonesia assessed that "In terms of the numbers killed the anti-PKI massacres in Indonesia rank as one of the worst mass murders of the 20th century...".