The Thirtieth of September Movement (Indonesian: Gerakan 30 September, abbreviated as G30S, also known by the acronym Gestapu for Gerakan September Tiga Puluh, 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'tat, resulting in the unofficial but more accurate name of Gestok, for Gerakan Satu Oktober, or First of October Movement. 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.

In the days and weeks that followed, the army, socio-political, and religious groups blamed the coup attempt on the Communist Party of Indonesia (PKI). Soon a mass purge was underway, which resulted in the imprisonment and deaths of actual or suspected Communist Party members and sympathizers. Under the New Order, the movement was usually referred to as "G30S/PKI" by those wanting to associate it with the PKI, and this term is also sometimes used by the current government.

Investigations and questioning of Suharto's version of the events were long obstructed in Indonesia. While the CIA initially believed that Sukarno orchestrated all of it, several outside sources found inconsistencies and holes in the army claims, notably Benedict Anderson and Ruth McVey who wrote the Cornell Paper that challenged it.

Suharto (; Indonesian pronunciation ; 8 June 1921 – 27 January 2008) was an Indonesian army officer and politician, who served as the second and the longest serving president of Indonesia. Widely regarded as a military dictator by international observers, Suharto led Indonesia as an authoritarian regime for 31 years, from the fall of Sukarno in 1967 until his resignation in 1998. The legacy of his 31-year rule, and his US$38 billion net worth, is still debated at home and abroad.Suharto was born in the small village of Kemusuk, in the Godean area near the city of Yogyakarta, during the Dutch colonial era. He grew up in humble circumstances. His Javanese Muslim parents divorced not long after his birth, and he lived with foster parents for much of his childhood. During the Japanese occupation era, Suharto served in the Japanese-organized Indonesian security forces. During Indonesia's independence struggle, he joined the newly formed Indonesian Army. There, Suharto rose to the rank of Major general by the time full Indonesian independence was achieved.

An attempted coup on 30 September and 1 October 1965 was countered by Suharto-led troops. According to the official history made by the army, this attempt was backed by the Communist Party of Indonesia (PKI). The army subsequently led a nationwide violent anti-communist purge and Suharto wrested power from Indonesia's founding president, Sukarno. He was appointed acting president in 1967 and elected president the following year. He then mounted a social campaign known as "de-Sukarnoization" to reduce the former president's influence. Support for Suharto's presidency was active throughout the 1970s and 1980s. By the 1990s, the New Order's increasing authoritarianism and widespread corruption were a source of discontent and, following the 1997 Asian financial crisis which led to widespread unrest, he resigned in May 1998. Suharto died in January 2008 and was given a state funeral.

Under his "New Order" administration, Suharto constructed a strong, centralised and military-dominated government. An ability to maintain stability over a sprawling and diverse Indonesia and an avowedly anti-communist stance won him the economic and diplomatic support of the West during the Cold War. For most of his presidency, Indonesia experienced significant industrialisation, economic growth, and improved levels of educational attainment. Plans to award the status of National Hero to Suharto are being considered by the Indonesian government and have been debated vigorously in Indonesia. According to Transparency International, Suharto is one of the most corrupt leaders in modern history, having embezzled an alleged US$15–35 billion during his rule.