After leaving the country, he traveled for years—earning money as a sailor, ship's stoker, house painter in France, ski instructor, and professional boxer—across the Mediterranean, the Middle East, and North Africa, all before the outbreak of World War II.

Upon returning home, he enrolled in the University of Ljubljana's Faculty of Engineering, which he did not graduate from, and read medical textbooks in an attempt to better understand his emotional condition.

[3] After the Axis invasion of Yugoslavia in April 1941, as a member of the Sokol athletic movement he joined the Liberation Front and participated in its underground activities in the annexed Province of Ljubljana until the authorities sent him to the Gonars concentration camp in 1942.

He gave an idiosyncratic description of the years he spent with the Slovene Partisans in his 1975 novel Menuet za kitaro (Minuet for Guitar), described the ruthless environments in repressive institutions, such as the army and the prison in the 1982 novel Levitan, and described the period before and during World War II in the third part of his trilogy, Komedija človeškega tkiva (A Comedy of Human Tissue).

The Yugoslav critics were part of official Titoist nomenclature, and rejected his bohemian style and freethinking attitude and accused his writings of being decadent, cynical, and a glorification of evil, amorality, and nihilism.

Alternative Slovene writers and literary thinkers, such as Dušan Pirjevec Ahac and Taras Kermauner, were influenced by Zupan's work and they challenged the Titoist cultural policies.