The Madiun Affair, known locally as the Communist Party of Indonesia rebellion of 1948, was an armed conflict between the government of the self-proclaimed Republic of Indonesia and the left-wing opposition group Front Demokrasi Rakyat during the Indonesian National Revolution. The conflict began on September 18, 1948, in Madiun, East Java, and ended three months later when most FDR leaders and members were detained and executed by TNI forces.

The Communist Party of Indonesia was a communist party in the Dutch East Indies and later Indonesia. It was the largest non-ruling communist party in the world before its violent disbandment in 1965. The party had two million members in the 1955 elections, with 16 percent of the national vote and almost 30 percent of the vote in East Java. During most of the period immediately following the Indonesian Independence until the eradication of the PKI in 1965, it was a legal party operating openly in the country.