30 September Movement

The Thirtieth of September Movement (Indonesian: Gerakan 30 September, abbreviated as G30S, also known by the acronym Gestapu for Gerakan September Tiga Puluh or sometimes called Gestok, for Gerakan Satu Oktober, First of October 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 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 at least. 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.

In the days and weeks that followed, the army, socio-political, and religious groups pointed the coup attempt on the Indonesian Communist Party (PKI). Soon a mass purge was underway, which resulted in the imprisonment and death of alleged communists and communist sympathizers. Under the "New Order" regime, and sometimes used by the current government, the movement was usually referred to as "G30S/PKI" by those wanting to associate it with the PKI.

From 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-imperial ideology saw Indonesia increasingly dependent on the Soviet Union and China. By 1965, at the height of the Cold War, the PKI penetrated all levels of government extensively. 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 overlapping with the PKI.

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