The Madiun Affair was a communist-led uprising in 1948 during the Indonesian National Revolution in the town of Madiun, which formed part of a broader conflict that had between the left and right wings of the Republican movement, especially around the issue of the demobilisation of popular militia. Its quashing was a major blow to the communist party (PKI) in particular.
On 18 September 1948 an 'Indonesian Soviet Republic' was declared in Madiun, in the western part of East Java, by members of the Indonesian Communist Party (PKI) and the Indonesian Socialist Party (PSI). Judging the time as right for a proletarian uprising, they intended it to be a rallying centre for revolt against "Sukarno-Hatta, the slaves of the Japanese and America". The rebellion was put down within a few weeks, although the rebels had in the meantime killed East Java's governor R. M. Suryo, pro-independence doctor Moewardi, as well as several police officers and religious leaders.
Tens of thousands of people were killed and imprisoned as part of the rebellion and its aftermath. Many of the PKI key leaders were executed, including party chairman Musso (recently returned from exile in the Soviet Union) and former Republic of Indonesia Prime Minister, Amir Sjarifuddin. More than 30,000 left-wing cadre were imprisoned.
The quashing of the rebellion turned vague American sympathies towards Indonesian independence into diplomatic support. Internationally, the Republic was now seen as being staunchly anti-communist and a potential ally in the brewing global Cold War between the American-led 'free world' and the Soviet-led bloc.