The Bulgarian occupation of Serbia during World War I started in Autumn 1915 following the invasion of Serbia by the combined armies of Germany, Austria-Hungary and Bulgaria. After Serbia's defeat and the retreat of its forces across Albania, the country was divided into Bulgarian and Austro-Hungarian occupation zones.

The Bulgarian occupation zone extended from modern-day Southern and Eastern Serbia, Kosovo and North Macedonia. The civilian population was exposed to various measures of repression, including mass internment, forced labor, and a Bulgarisation policy. According to academic Paul Mojzes: "it appears that ethnic cleansing (at a minimum) and genocide (at the maximum) did take place between 1915 and 1918", what historian Alan Kramer has termed a: "dynamic of destruction".

The occupation ended in late September 1918, after the Allied offensive at Dobro Polje, spearheaded by Serbian and French forces, pierced the Bulgarian front and liberated Serbia.

Background Bulgaria war aims In the wake of the 1878 Treaty of San Stefano, Bulgarian leaders aspired to reconstitute Greater Bulgaria, making the Serbian regions of Pomoravlje and Macedonia preoccupations of Bulgarian irredentism. After its defeat in the Second Balkan War in 1913, the Kingdom of Bulgaria had to limit its ambitions over Macedonia. With Serbia trying to obtain sea access through Albania, the Austro-Hungarians took measures to establish a strong border between Albania and Montenegro. With the Florence Protocol in December 1913, Bulgaria renounced its claims on Serbian Macedonia, which was annexed to Serbia.

The Entente had been pressing Bulgaria to join them; Bulgaria's price was the acquisition of the long-coveted region of Macedonia from Serbia and Greece. The Entente regarded this as reasonable on ethnic grounds but it was strongly opppsed by both Serbia and Greece. The Central Powers, however, were prepared to cede Serb and Greek territory to Bulgaria should the Bulgarians join them. Bulgaria's traditional aims lay in the Bulgarian-inhabited areas of Macedonia, Dobrudja, and European Turkey, but in 1915 it demanded territory well beyond its ethnographic borders. On 6 September 1915, the Bulgarian government joined the Central Powers upon signing a secret treaty of alliance with Germany.

Invasion of Serbia On 6 October 1915 under the overall command of German General August von Mackensen, Austria-Hungary and Germany began the fourth invasion of Serbia since the beginning of the war. On 14 October, the Bulgarian armies moved into Serbian territory joining the ongoing invasion. Bulgaria entered the war on the side of the Central Powers, with the primary goal of regaining territory briefly gained from the Ottoman Empire in 1912–13, then lost to Serbia in 1913. The pressure of Austro-Hungarian, Bulgarian and German armies in the north, and their massive superiority in numbers and equipment, forced the Serbs to withdraw across northern and central Albania. On 28 November 1915, Army Group Mackensen announced the end of the Serbian campaign, thereby ending the offensive.

The Kingdom of Serbia was overrun after the six-week campaign. It was then divided between Austria-Hungary and Bulgaria. In early 1916, regions in the west and north and part of Kosovo were also ceded to Austria-Hungary. Germany established its own occupation zone in the area east of Velika Morava, Južna Morava in Kosovo and the Vardar valley. The Germans took control of railways, mines, forestry, and agricultural resources. The agreement of 6 September granted Bulgaria the whole of Macedonia and Eastern and Southern Serbia, from the Danube to Kosovo in the south. The new border with Austria-Hungary ran along the Morava river to Stalać and then between the South (Južna) and West (Zapadna) Morava rivers, the region of Skopska Crna Gora and Šar Planina mountain. Austro-Hungary took the rest of Serbia. The Bulgarians divided the territories occupied by their troops into two military general governorates.

Bulgarian hegemony

Occupation zones Two administrative zones supervised by a military commander were created: Military Inspection Area of Morava: The zone for Serbia with its command in Niš, it encompassed the territories of Eastern and Southern Serbia, (as laid down in the secret treaty between Bulgaria and Germany of 6 September 1915), which meant the Južna Morava river valley east of the Morava river, divided into six districts and the Pirot area. Military Inspection Area of Macedonia: The zone encompassing Macedonia, with its center in Skopje; the greater part of Kosovo field – Pristina, Prizren, Gnjilane, Urosevac, Orahovac was also placed in that zone; the Bulgarians intended to include all of Kosovo and even parts of Albania occupied by their troops into that zone, in the spring of 1916, this nearly resulted in armed conflict between Bulgarian and Austrian forces.

System of occupation Bulgarian policy in Macedonia, and to some degree in occupied Serbia, was motivated by what historian Alan Kramer has termed a 'dynamic of destruction' a desire not just to defeat the enemy militarily, but also to erase all traces of its culture and destroy any evidence that it had ever been there at all. To create pure Bulgarian territories, the Bulgarian military government started implementing in eastern Serbia, Macedonia, and parts of Kosovo a political system of systematic denationalization, Bulgarisation, and economical exploitation.

In the Morava zone, where the majority of the population was Serbs, transforming the region into a part of the Kingdom of Bulgaria, meant the extermination of the Serbian nation and culture and for this the removal of all representatives of Serbian national spirit; teachers, clergymen, journalists as well as members of Serbian Parliament as well as former soldiers, officers and military official between 18 and 50 years of age were interned, shot or deported to Bulgaria as prisoners of war or to work as forced labourers.

In the zone of Macedonia, Bulgaria, like Serbia, did not recognise the local Slav population as a separate ethnic or national group. Both Bulgaria and Serbia considered the Slavic-speaking population as being ethnically linked to their nation and thus asserted the right to seek their integration. The Bulgarian denationalisation policy, including its paramilitary aspect, was almost identical in its intent and execution to the Serbianisation policy that preceded it in the contested region between the two countries. About half of Vardar Macedonia, as the region was called by Serbia, was also inhabited by various ethnic groups who did not identify as Bulgarian; according to a 1912 survey by the British Foreign Office, namely Serbs, Turks, Albanians, Greeks, Vlachs, Jews and Gypsies. In the eastern parts of the region, where a considerable part of Macedonian Slavs had pro-Bulgarian sentiments or felt themselves to be Bulgarians, that population welcomed the Bulgarian army as liberators. For the rest of the population and in particular for the Macedonian Slavs who identified as Serbs (or those who felt neither Serb nor Bulgarian), the brutality of the Bulgarian army, the irregulars Komitadji and the later civil administration had all the features of ethnic cleansing.

Role of paramilitaries

Besides the regular army, Bulgaria's paramilitary groups played an immense part in the fighting capabilities of Bulgaria, they were used as auxiliaries to provide knowledge of local conditions. They were known as komitadjis, these irregular troops also contributed strongly in brutalising the war. The notorious Internal Macedonian Revolutionary Organization (IMRO) served as a gendarmerie working hand in glove to Bulgarianise the region. During the war the IMRO arose from a clandestine organization into an important factor of the Bulgarian nationalistic policy supporting the Bulgarization of the area.

Some paramilitary companies joined the Bulgarian Army forming the 11th Macedonian Infantry Division. Additionally, this division had guerrilla companies formed by IMRO-irregulars, that participated at the beginning of 1916 in several massacres of Macedonian Serbs in the areas of Azot, Skopska Crna Gora and Poreče. After the massacres, the Military police chief informed the Minister of Internal Affairs that he could not cope with the lawlessness of the paramilitaries.Величко Георгиев, Стайко Трифонов (съставители). История на българите в документи 1878 – 1944. Т. II: Периодът на войните 1912 – 1918. София, Просвета, 1996. , стр. 490-491. Regular Bulgarian troops took control of the region while komitadjis were appointed mayors and prefects and took control of the whole police structure. Every major town was controlled by a komitadji leader whose power became absolute and legitimized through a new administrative system. IMRO member Naum Tomalevski, whose house was the headquarters of the Ilinden Uprising of 1903, was appointed mayor of Kruševo. After 1917 the Bulgarian government started using paramilitary groups to gain control over the internal situation in both Pomoravlje and Macedonia. Aleksandar Protogerov who headed the Bulgarian occupation troops in the Morava region crushed the uprising in the Toplica district with the help of IMRO irregulars. Bulgarians paramilitary groups were responsible for multiple instances of war crimes committing during the war in the parts of the Kingdom of Serbia under Bulgarian occupation.

War crimes

Counter-insurgency operations

Serbian Uprising

From October 1916 to February 1917, a spontaneous Serbian uprising broke out in the Bulgarian-occupied territories of southern and eastern Serbia, notably in the valley of the Južna Morava, on Mount Kapaonik and in Kosovo. It followed attempts by the Bulgarian army to force draft Serbian men into the Bulgarian army and shoot those who resisted. The scheme was identical to that previously pursued by the Serbian army, which in August 1914 had attempted to conscript more than 60,000 people from Macedonia on the frontline defense of Serbia against the Austria-Hungarian attack.

Chetnik Lieutenant Kosta Milovanović-Pećanac was flown by the Serbian supreme command southwest of Niš to locate and organise rebel action against enemy lines of communication, in preparation for a Serbian army offensive from the Salonika front. Most Serbian guerrilla leaders, including both Kosta Vojinović and Kosta Pećanac, set up their headquarters on Mount Kapaonik. By 3 March guerilla units made of local inhabitants had liberated Kuršumlija and Prokuplje as well as regions in the areas of Vlasotince and Sokobanja, coming 9 km southwest of Niš. General Adolf Baron von Rhemen, governor of the Military General Governorate of Serbia, ordered Austro-Hungarian troops to cross into the Bulgarian zone in support while requesting the Austrian Supreme Command to send assistance. Three armies of around 30,000 men tasked with crushing the uprising were sent to the region in the largest-ever anti-guerilla campaign. Austro-Hungarian, German and Bulgarian troops were brought from the Macedonian and Italian front, while in the Mitrovica district, Albanian gendarmes were sent to track down rebel leaders. On 8 March, the Bulgarian army started their offensive while supported by Albanian detachments, artillery and air support. The Austro-Hungarians launched their attack on 12 March 1917.