Princen was fully committed to his new cause, seeing front-line service under Kemal Idris and taking part in the fighting retreat of the Siliwangi Division under then-Colonel A. H. Nasution, from Central Java to "guerrilla cantons" established in West Java – an action which came to be known as the Long March Siliwangi (derived from the famed Long March of Mao Zedong's Chinese Communist Party). He was appointed staff officer in the Second Brigade of Grup Purwakarta, active in the environs of the city of Purwakarta.