Following the German-led Axis invasion of Yugoslavia in April 1941, the Ustaše implemented genocidal policies against the NDH's Serbs, Jews and Roma. Drenović joined the Partisans and distinguished himself during the initial uprising against the NDH government by capturing the town of Mrkonjić Grad in August 1941. He was then appointed to command the 3rd "Petar Kočić" Battalion in central Bosnia and was appointed the deputy commander of the 3rd Krajina Detachment.