During the World War II, he joined the anti-fascist movement led by the Communist Party of Yugoslavia (KPJ).
[1] During the Croatian Spring, Šibl supported the reformist leadership of the Socialist Republic of Croatia led by Savka Dabčević-Kučar and Miko Tripalo.
[2] Following the defeat of Dabčević-Kučar and Tripalo and suppression of Croatian Spring in late 1971, Šibl was removed from all party and state functions.
Šibl's principal literary works are Zagrebačka oblast u narodnooslobodilačkoj borbi ("Zagreb District in the People's Liberation Struggle"), Iz ilegalnog Zagreba 1941 ("From Clandestine Zagreb 1941"), Partizanski razgovori ("Partisan Talks"), Ratni dnevnik ("Wartime Diary"), and Sjećanja ("Memories").
[1] Lifetime achievement award by the Croatia's public television and radio broadcasting company Hrvatska radiotelevizija is named after Šibl.