During March 1942 Velebit left Zagreb and joined the Partisans who mounted a guerrilla resistance to the Nazis and domestic collaborationists. Right away Tito included him in the army's Supreme Command where he mostly worked on establishing some sort of military court authority. Due to his education and knowledge of foreign languages, along with Koča Popović and Milovan Đilas, Velebit was part of the Partisan delegation in Gornji Vakuf and Zagreb at the controversial March 1943 German-Partisan negotiations while the Battle of Neretva raged several hundred kilometers to the south. Velebit and Đilas conducted the negotiations under pseudonyms Vladimir Petrović and Miloš Marković, respectively, while Koča Popović gave his real name.