From Russia’s Last Gasp: The Eastern Front 1916–17, by Prit Buttar (Osprey, 2016), Kindle Loc. 672-92:
As soon as the fighting men had moved on, occupation authorities began their work. Bulgaria intended that the territory it gained from Serbia would become completely Bulgarian in character. Accordingly, all schools in the Bulgarian zone were required to teach exclusively in Bulgarian, and thousands of Serbian males were arrested in an attempt to reduce the risk of resistance. Officially, they were interned, but the reality was rather different, as an Austro-Hungarian officer reported:
It is known that most of the Serbian intelligentsia, i.e. administrators, teachers, clergy and others, withdrew with the remnants of the Serbian Army, but some have gradually begun to return for personal or material reasons. Here, in occupied territory, it is virtually impossible to find either them or those who did not flee; they have ‘gone to Sofia’ as the new Bulgarian saying goes. These men are handed over to Bulgarian patrols as suspects without any due legal process, with orders that they should be ‘taken to Sofia’. The patrols actually return the following day without them. Whether they are taken 20 or 200km [12 or 120 miles] it is all the same. The patrols take up shovels, disappear into the mountains, and soon return without the prisoners. Bulgarian officers do not even try to conceal the executions, but boast about them.
Whilst such killings were shocking, even to the Austro-Hungarian officer who reported them, they were not unusual for the region. After Serbia seized territory from the Turks during the First Balkan War, Serbian irregulars had carried out many such killings, not stopping with the intelligentsia. 32 During the invasions of 1914, the k.u.k. Army had also committed many atrocities, and after the 1915 invasion there was widespread internment in the area under Austro-Hungarian control, though fewer killings than in 1914. Nevertheless, there were summary executions at the hands of the Austro-Hungarian authorities with little or no legal process. Many of those interned became ill or died as a result of poor housing and inadequate food, and those who were not actually ill were frequently used as forced labour. As was the case in the Bulgarian zone of occupation, schools used the language of the occupiers.
Such policies, designed to crush Serbian national consciousness, had severe effects on productivity in a land already badly scarred by war. Agricultural production plummeted due to the absence of so many men from the countryside; in an attempt to make the conquered land more productive, both Bulgarian and Austro-Hungarian authorities resorted to harsher measures, and inevitably these merely resulted in further resentment and even lower production.