As a fierce opponent of Yugoslav centralism and nationalism, Kozak was among the founders of the left liberal journal Sodobnost, which he edited together with Josip Vidmar, Fran Albreht and Stanko Leben.
In a period when the Communist party was outlawed in the Kingdom of Yugoslavia, the magazine enabled many prominent Communists to publish articles under pseudonyms; among them were Edvard Kardelj, Boris Kidrič, and Ivo Brnčič.