Short answers, pulled from the story.
The Communist Party of Indonesia traces its origins to 1914 when Dutch socialist Henk Sneevliet founded the Indies Social Democratic Association in Surabaya. The organization initially included only three Indonesian members among eighty-five total participants before evolving into a communist party.
Government forces crushed revolts that began on the 12th of November 1926, in Batavia, Padang, Bantam, and Surabaya within days or weeks. Colonial authorities arrested thirteen thousand people resulting in four thousand five hundred imprisonments, one thousand three hundred internments, and eight hundred twenty-three exiles to the Boven-Digoel camp in Western New Guinea.
Membership jumped from five thousand in 1950 to one hundred sixty-five thousand by 1954 and reached about three million by 1965. This growth made it the strongest communist party outside China and the Soviet Union with front organizations claiming one-fifth of the Indonesian population.
General Suharto took control of the army and put down the abortive coup by October 2 while blaming the Communist Party of Indonesia for the assassinations despite inconclusive evidence. An estimated five hundred thousand communists and suspected sympathizers died during the purge initiated after the night of September 30 and October 1.
The United States supplied economic technical and military aid when killings began and embassy officials provided kill lists containing names of thousands of suspected high-ranking PKI members. Declassified diplomatic cables released in 2017 corroborated that American backed the Indonesian military knowing they embarked upon mass killings.