 In order to get to Bihać , Đurišić made a safe @-@ conduct agreement with elements of the Armed Forces of the Independent State of Croatia ( HOS ) and with the Montenegrin separatist Sekula Drljević . The details of the agreement are not known , but it appears Đurišić , Ostojić and Baćović and their troops were meant to cross the Sava river into Slavonia where they would be aligned with Drljević as the " Montenegrin National Army " with Đurišić retaining operational command . The Chetniks however , appear to have tried to outsmart the HOS forces and Drljević by sending their sick and wounded across the river , but retaining their fit troops south of the river , after which they began moving them westwards . Harassed by both the HOS troops and Partisans , they reached the Vrbas river , which they began to cross . In the Battle of Lijevče Field , north of Banja Luka , the combined Chetnik force was badly beaten by a strong HOS force which possessed German @-@ supplied tanks . 
