 After the fall of Belgrade on 20 October 1944 , Mihailović and a force of a few hundred of his Chetniks withdrew from Serbian territory , crossed the Drina and based themselves in the Majevica mountain area north of Tuzla in north @-@ eastern Bosnia . Ostojić , who was commanding Herzegovinian Chetniks , was joined by Đurišić and his Montenegrin Chetniks and together they withdrew towards Mihailović . 
