Supardjo was a Brigadier General in the Indonesian Army. He was one of the leaders of the 30 September Movement, a group that killed six of the army's top generals and launched a failed coup attempt on 30 September 1965.
Supardjo was regimental commander of the Siliwangi Division which was stationed in West Java. As a result of his pro-Communist sympathies and actions, he was sent to Kalimantan, away from the centres of power on Java, and took part in the Indonesian actions against British and Malaysian troops in Borneo (see Konfrontasi). Based at Menggaian in West Kalimantan, he led the Fourth Combat Command of KOSTRAD, the army's strategic reserve.
On 28 September 1965, he left his post without the knowledge of then KOSTRAD commander, Suharto (later President of Indonesia). He had received a telegram from his wife stating that his child was sick. At Supardjo's trial in 1967, an ex-Communist official testified that the cable was code between Supardjo and Communist participating in the coup, and that the real reason for his return to Java was to help command the coup.
Some Supardjo's military colleagues reported he was resentful over his slow pace of promotion and may have had a grudge against army commander, General Yani, who was one of the 30 September Movement's principal victims. According to General Sarwo Edhie Wibowo, commander of the fiercely anti-communist RPKAD para-commandos, Supardjo requested RPKAD reinforcements be sent to Kalimantan on 1 October. When Sarwo Edhie heard of the coup attempt, the sailing orders to Kalimantan were cancelled.