 After his return to Yugoslavia, he settled in Ljubljana, Slovenia, and became a professional historian, focusing on the history of the worker movement in Slovenia, and on the history of the Communist Party.Together with Dušan Kermavner, he was one of the foremost Communist historians of the Socialist movement in Slovenia.He was also vocal in the defence of the federalist system in Yugoslavia and of the autonomy of the single Yugoslav republics against centralist pressures.