From the late 1950s, President Sukarno's position came to depend on balancing the opposing and increasingly hostile forces of the army and the PKI. His "anti-imperialist" ideology made Indonesia increasingly dependent on the Soviet Union and, particularly, China. By 1965, at the height of the Cold War, the PKI extensively penetrated all levels of government. With the support of Sukarno and the air force, the party gained increasing influence at the expense of the army, thus ensuring the army's enmity. By late 1965, the army was divided between a left-wing faction allied with the PKI and a right-wing faction that was being courted by the United States.

In need of Indonesian allies in its Cold War against the Soviet Union, the United States cultivated a number of ties with officers of the military through exchanges and arms deals. This fostered a split in the military's ranks, with the United States and others backing a right-wing faction against a left-wing faction leaning towards the PKI.

The Thirtieth of September Movement was a self-proclaimed organization of Indonesian National Armed Forces members who, in the early hours of 1 October 1965, assassinated six Indonesian Army generals in an abortive coup d'état. Later that morning, the organisation 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.