Summary from Wikipedia

He was born in Vukova Gorica, then he lived in Austria-Hungary. He finished grammar and high school in Zagreb, and enrolled onto the Faculty of Theology at the University of Zagreb. During the First World War, he was drafted into Austro-Hungarian Army where he took part in the fighting against Serbian forces on the Drina River. Later he was sent to the Eastern Front where he used the opportunity to defect to the Russians. From there he joined the Yugoslav volunteers fighting within the Serbian army on the Salonica front.