It was popular among young intellectuals and students, as well as members of the underground movements led by the two men during the Japanese occupation of the Dutch East Indies.

[2] Both Sjahrir and Amir served terms as prime minister, while other Socialist Party members held senior cabinet posts.

[4][5] Joining him in the new party were Lintong Mulia Sitorus and other followers who agreed with his position on nationalism and not having a strongly pro-Soviet line.

This was followed by a military crisis brought about by army chief-of-staff General Nasution's plan to implement widespread transfers of officers, many of whom had built up private business interests in the regions.

It was headed by Amir Sjarifoeddin as prime minister and included Burhanuddin Harahap from the Masyumi Party and Sumitro Djojohadikusumo.

Hatta and PSI leaders in Jakarta called for a negotiated settlement, but the government decided on a military solution, which defeated the rebellion by mid-1958.

Finally, in August 1960, President Sukarno formally banned the PSI and the Masyumi Party for their opposition to Guided Democracy and their alleged support of the PRRI rebellion.

It stressed the need for modernization, economic development and rational planning and organization, while opposing extreme nationalism and anti-foreign sentiment.