[4][6] During the Japanese occupation of the Dutch East Indies, having recently graduated from law school, Sitorus got a low-level position in the Ministry of Justice along with his former schoolmates Ali Boediardjo and Andi Zainal Abidin.
[9] The three read widely during that period and regularly attended Sjahrir lectures; other Batak youth nicknamed them De Drie Musketiers (the three musketeers).
[10] As the Japanese occupation ended and Java was occupied by Allied Forces, Sitorus wrote for Sjahrir's new Dutch-language literary magazine Het Inzicht (The Insight) along with such figures as Chairil Anwar, Noegroho and Soedjatmoko.
Starting in the late 1940s he wrote for and occasionally edited the party's newspaper, Sikap, a role he held for the following decade.
[26][27] Before and especially after the 1955 election he was in favour of the PSI working more closely with President Sukarno; Sjahrir preferred that the party keep its distance.
[28][29] The party sent delegates to the second Asian Socialist Conference in Bombay in 1956; Sitorus and Sjahrir attended, as did fellow PSI members Soebadio Sastroatomo and Soedjatmoko.