The Communist Party of Indonesia (Dutch: Partai Komunis Indonesia) was borne in 1920 out of the Indonesian Social Democratic Association in Java. The PKI joined the Comintern in the same year it was formed, 1920. In its first years the PKI abandoned democratic centralism and gave a great deal of autonomy to its various cells. The PKI played a leading role in the national liberation uprising which lasted from 1926 to 1927. The uprising was defeated. PKI was declared illegal and the organization was destroyed. In 1935 PKI was recreated, this time as an underground organization.
During the Japanese occupation of 1942 to 1945, the PKI carried out an underground struggle under severe conditions. The communists of Indonesia took an active part in the national liberation revolution which led to the proclamation of Indonesia as an independent republic on August 17, 1945.
The PKI had become an influential political force. Its representatives became leaders in various parts of Indonesian society, including within the government. Amir Sjarifuddin, a member of the Politburo of the Central Committee of the PKI, was Minister of Defense from 1945 to 1947 and then became Prime Minister in 1947. At the 1948 PKI conference run by Musso, the resolution "New Road for the Republic of Indonesia" was accepted. The conference accepted its solution for how the PKI would work with and control the socialist and workers parties of Indonesia.
PKI suffered serious loss in September 1948 as a result of brutal actions in the city of Madiun in East Java, brought about by reactionary forces (the Madiun Affair). Many of the most capable PKI leaders were summarily executed including Musso and Amir Sjarifuddin. Nonetheless, by 1951 PKI had already bounced back from this tragedy. The PKI came under the leadership of D.N. Aidit, who helped lead the party back into legal political work. In 1951 the newspaper Harian Rakjat became PKI's official organ.
The 5th Congress of the KPI in 1954 resolved a course of building a mass party and creating a united national front of all patriotic forces in the country. KPI received more than six million votes during the first parliamentary elections in 1955, coming in fourth place with 16% of the vote. In 1958 local elections, KPI got eight million votes, more than any other party.
The 6th Congress of the KPI in 1959 resolved that the primary and most important task of the party was implementing the requirements, in the manner of the August 1945 revolution, to create a government of people's democracy - a government of the united national front, formed on the basis of the union of working class and peasantry, with the working class playing the leading role.
By the middle of the 1960's the KPI had more than 3.5 million members. KPI controlled trade-union, youth and other organizations which contained about 16 million people. KPI had the possibility of become the leading force in the Indonesian people for political and economic independence from imperialism and internation reaction.
KPI leadership had begun to come under the influence of Maoism in the early 1960's. The party size increased massively from the late 1950s to the early 1960s due to peasants and urban petit bourgeoisie joining the party. KPI leadership unconditionally supported the "guided democracy" concept of President Sukarno, which consolidated power in the President's hands, and adapted party policy to the policy of the President. The 7th Congress of the KPI in 1962 saw the replacement of class positions and proletarian slogans for more nationalistic ones. The leaders of KPI proclaimed the political program of Sukarno as the second program of the party. In the area of foreign policy, KPI leadership supported Sukarno's idea of a Jakarta-Peking axis, made propaganda with the Maoist concept of the special role of countries of the third world in liberation struggles and helped create self-contained Afro-Asian organizations which helped cut KPI ties to the orthodox communist movement.
1964 to 1965 was a period of economic instability in Indonesia, increasing dissatisfaction of the masses, and political isolation of the country. During this period, reactionary forces became active, including among high-ranking army officers. The plans by reactionaries to seize political authority caused a plot among left-wing junior army officers called the "September 30 movement". A small segment of KPI leadership was informed of this movement and supported it. The September 30 movement made a palace putsch, which did not receive widespread support from the masses. Right-wing groups united across the country and wreaked bloody terror against communists and other democrats. Several hundred thousand members of KPI and left-wing organizations were killed. Many leading party members were killed (D. N. Aidit, M. H. Lukman, Njoto, Njono etc.) and tens of thousands were arrested. In March of 1966 KPI and its affiliated public organizations were declared illegal. A law prohibiting the propagation of communist ideology was established. Failure of the September 30th Movement also ultimately led to the downfall of Sukarno and the passage of authority into the hands of the army leadership.
What was left of the KPI was deep underground. In the following years the leadership of what remained of the Indonesian communist movement issued self-criticism and documents assessing the current situation.