Dretar was born in Nova Gradiška, Croatia, as the son of Ružica Rivić from Ljubija Rudnik in Bosnia and Herzegovina and Vladimir Dretar from Cernik in Nova Gradiška. His mother was a clerk with a finished civil School for Catholic Girls, and the father, after completing the Higher Secondary School in Nova Gradiška enrolled the study of forestry at the Forestry Faculty at the University of Zagreb, but in 1941 he left for military education in Stockerau where he received the title of noncommissioned officer. He served as a noncommissioned officer in the Blue SS division from which he deserted because of the humiliating treatment of Croatians by the German commanders. In his absence, a military court sentenced him to death. He became an Ustasha volunteer, rising to the rank of second lieutenant. From there he deserted because of the inhumane behavior of the Ustashas and their commanders and, again, was sentenced to death in his absence. He settled down in the Croatian Home Guard as a lieutenant, he was station commander in Vrba. After the fall of Nova Gradiška he retreated with the rest of the Croatian Armed Forces and disappeared in Bleiburg.