 To reach Bihać , Đurišić made a safe @-@ conduct agreement with elements of the Armed Forces of the Independent State of Croatia ( NDH ) and with the Montenegrin separatist Drljević . The details of the agreement are not known , but it is thought he and his troops intended to cross the Sava river into Slavonia where they would join Drljević as the Montenegrin National Army , of which Đurišić was the operational commander . Đurišić apparently tried to outsmart them and sent only his sick and wounded troops across the river , keeping his fit troops south of the river . He began moving his command westwards ; harassed by the NDH troops and Partisans , Đurišić 's forces reached the Vrbas river north of Banja Luka in late May . Between 30 May and 8 April , the combined Chetnik force was defeated by a strong NDH force armed with German @-@ supplied tanks , in the Battle of Lijevče Field . This was probably the largest combat action between NDH forces and the Chetniks in the previous two years . 
