The ISDV began a Dutch-language publication, Het Vrije Woord (The Free Word, edited by Adolf Baars), in October 1915.

The ISDV became increasingly incompatible with the SDAP leadership in the Netherlands, who distanced themselves from the association and began to equate them with Volksraad (People's Council).

Around the same time, the ISDV and communist sympathizers began infiltrating other political groups in the East Indies in a tactic known as the "block within" strategy.

At its 23 May 1920 congress in Semarang, the ISDV changed its name to Perserikatan Komunis di Hindia (PKH; the Communist Union of the Indies).

Agus Salim, the organization's secretary, introduced a motion banning Sarekat Islam members from dual membership in other parties.

[11] At the Fifth Comintern Congress in 1924, it was emphasized that "the top priority of communist parties is to gain control of trades unions"; there could be no successful revolution without this.

[12] In a May 1925 plenary session, the Comintern executive committee ordered the Indonesian communists to form an anti-imperialist coalition with non-communist, nationalist organizations; extremist elements led by Alimin and Musso called for a revolution to overthrow the Dutch colonial government.

Tan Malaka, the Comintern's agent for Southeast Asia and Australia, did not agree with the plot (partly because he believed that the PKI had insufficient mass support).

Many non-communist political activists were also targeted by colonial authorities under the pretext of suppressing the communist rebellion, and the party was outlawed by the Dutch East Indies government in 1927.

Although PKI militias played an important role in fighting the Dutch, President Sukarno was concerned that the party's growing influence would eventually threaten his position.

Because the PKI's growth troubled the right-wing sectors of Indonesian society and some foreign powers (especially the vigorously anti-communist United States), its relationship with the other forces also fighting for independence was generally difficult.

It was claimed by army sources that the PKI had announced the proclamation of a Soviet Republic of Indonesia on 18 September, with Musso as president and Amir Sjarifuddin as prime minister.

During the 1950s, the party began publishing again; its main publications were Harian Rakyat and Bintang Merah, as well as quarterly journal PKI dan Perwakilan.

[18] The PKI led a series of militant strikes in August 1951 which were followed by clamp-downs in Medan and Jakarta, and the party leadership briefly went underground.

Also under Aidit, the PKI began to consider the possibility of cooperation with the Indonesian National Party (PNI) to overthrow the Masyumi-led cabinet of Mohammad Natsir.

[3] Opposition to continued Dutch control of Irian Jaya was often raised by the party during the decade, and the PKI office in Jakarta experienced a grenade attack in July 1957.

The revolutionary government immediately began arresting thousands of PKI members in areas under their control, and the party supported Sukarno's efforts to quell the rebellion (including the introduction of martial law).

Although the PKI supported Sukarno, it retained its political autonomy; in March 1960, the party denounced the president's undemocratic handling of the budget.

With growing popular support and a membership of about three million by 1965, the PKI was the strongest communist party outside China and the republics of the Soviet Union.

[23] Evidence linking the PKI to the generals' assassinations is inconclusive, leading to speculation that their involvement was very limited or that Suharto organised the events (in whole or in part) and scapegoated the communists.

[29][30][31][32][33][34] A tribunal held in the Hague in 2016 concluded the massacres were crimes against humanity, and the U.S. backed the Indonesian military "knowing well that they were embarked upon a programme of mass killings".

[5][36] According to UCLA historian Geoffrey B. Robinson, the Indonesian army's campaign of mass killings would not have occurred without the support of the US and other powerful Western governments.

Armed with wide-bladed knives called parangs, Moslem bands crept at night into the homes of communists, killing entire families and burying their bodies in shallow graves.

The murder campaign became so brazen in parts of rural East Java, that Moslem bands placed the heads of victims on poles and paraded them through villages.

The killings have been on such a scale that the disposal of the corpses has created a serious sanitation problem in East Java and Northern Sumatra where the humid air bears the reek of decaying flesh.

Part of the anti-Communist force responsible for the massacres consisted of members of the criminal underworld who were given permission to engage in acts of violence.

He attempted to rebuild the party on a base of interlocking groups of three members but made little progress before he was captured in December 1966[45] and sentenced to death in 1967.

According to Australian political scientist Harold Crouch, it was unlikely that the killings of NU members in Blitar had been ordered by the PKI leaders.

[46] Some party members were outside Indonesia at the time of the 30 September events; a sizeable delegation had traveled to the People's Republic of China to participate in the anniversary celebration of the Chinese Revolution.

During his presidency, Abdurrahman Wahid invited PKI exiles to return to Indonesia in 1999 and proposed the removal of restrictions on the open discussion of communist ideology.