Austria-Hungary declared war on Serbia on 28 July, marking the beginning of
World War I. Serbia was invaded by a combined German and Austro-Hungarian force on 7 October 1915. On 14 October, the
Kingdom of Bulgaria declared war on Serbia and invaded the country from the east. The
Serbian Army was forced to
retreat through Albania. Serbia was divided between the Austro-Hungarians, Germans and Bulgarians. The Bulgarian occupation zone was located in the area between the cities of
Skopje and
Niš, which had been a target of
Bulgarian nationalism. As Bulgarians emphasize, before 1878, that area was part of the
Bulgarian Morava and under the jurisdiction of the
Bulgarian Exarchate and had certain
Bulgarophile intelligentsia, but afterwards it was ceded to Serbia and pro-Serbian sentiments became prevailing ubiquitously. A policy of
Bulgarianisation targeting ethnic
Serbs was implemented there. As result in September 1916, the Serbian high command sent
Kosta Pećanac in the
Toplica District to organize a guerrilla uprising. There, Pećanac contacted several groups and joined forces with local leaders. As a consequence, one of the first measures undertaken by the Bulgarian military authorities was the mass-deportation of non-Bulgarian adult males. On 16 December 1916, the Bulgarian military governor of the occupied Serbian territories ordered that "all men between 18 and 50 who have served in the Serbian Army, all officers, former teachers, priests, journalists, former deputies, military functionaries, and all suspected persons, should be arrested and interned". Arrests of Serbian men followed. In January–February 1917, the Bulgarians began conscripting local Serbs for military service and a rumor was spread that the Allies had reached Skopje, so the Serbs should rise in revolt. The decision for this rebellion was taken and on 21 February, and the
Toplica rebellion broke out. Its leaders gathered several hundreds of rebels who conquered Prokuplje and Kuršumlija. Pećanac also attempted to attract Albanians on his side, but without success. On 12 March, the Bulgarian counterattack started under the command of
Alexander Protogerov involving
comitadjis' forces of the
Internal Macedonian Revolutionary Organisation. After several days of fighting, the Bulgarians entered
Prokuplje on 14 March and Austro-Hungarians the
Kuršumlija. As of 25 March, the order there was fully restored. In the battles, several thousand people were killed, including civilians. In April 1917, the Serbian guerrillas attacked a railway station and on 15 May, Pecanac entered the old Bulgarian border and invaded the town
Bosilegrad, which was inhabited by Bulgarians, and burned it down. Then he withdrew to
Kosovo, controlled then by the Austro-Hungarians. ==Massacre==