 Although the Serbian Army had put up fierce resistance and inflicted heavy casualties on the Austro @-@ Hungarians , Putnik became concerned that his lines were over @-@ extended . He began contemplating another strategic withdrawal , one in which Belgrade would have to be evacuated . On the night of 26 – 27 November , the Austro @-@ Hungarian 6th Army attacked all along the front and pushed deeper into the Serbian interior . 
