Vardar Macedonia in the context of "Kingdom of Serbia"

Central Serbia (Serbian: централна Србија, romanized: centralna Srbija), also referred to as Serbia proper (Serbian: ужа Србија, romanized: uža Srbija), is the region of Serbia lying outside the autonomous province of Vojvodina to the north and the disputed Kosovo region to the south. Central Serbia is a term of convenience, not an administrative division of Serbia as such, and does not have any form of separate administration.

Broadly speaking, Central Serbia is the historical core of modern Serbia, which emerged from the Serbian Revolution and subsequent wars against the Ottoman Empire. In the following period, Serbia gradually expanded south, acquiring Southern Serbia, Kosovo, Sandžak, and Vardar Macedonia, and in 1918 – following the unification and annexation of Montenegro and unification of Austro-Hungarian areas left of the Danube and Sava (Vojvodina) – it merged with other South Slavic territories into the Kingdom of Yugoslavia. The current borders of Central Serbia were defined after World War II, when Serbia became a republic within the Socialist Federal Republic of Yugoslavia, with Kosovo and Vojvodina as its autonomous provinces.

The history of Bulgaria during World War II encompasses an initial period of neutrality until 1 March 1941, a period of alliance with the Axis powers until 8 September 1944, and a period of alignment with the Allies in the final year of the war. With German consent, Bulgarian military forces occupied parts of the Kingdoms of Greece and Yugoslavia which Bulgarian irredentism claimed on the basis of the 1878 Treaty of San Stefano. Bulgaria resisted Axis pressure to join the war against the Soviet Union, which began on 22 June 1941, but did declare war on Britain and the United States on 13 December 1941. The Red Army entered Bulgaria on 8 September 1944; Bulgaria declared war on Germany the next day.

As an ally of Nazi Germany, Bulgaria participated in the Holocaust, contributing to the deaths of 11,343 Jews from the occupied territories in Greece and Yugoslavia. Though its native 48,000 Jews survived the war, they were subjected to discrimination. However, during the war, German-allied Bulgaria did not deport Jews from the core provinces of Bulgaria. Bulgaria's wartime government was pro-German under Bogdan Filov, Dobri Bozhilov, and Ivan Bagryanov. It joined the Allies under Konstantin Muraviev in early September 1944, then underwent a coup d'état a week later, and under Kimon Georgiev was pro-Soviet thereafter.

Old Serbia (Serbian: Стара Србија, romanized: Stara Srbija) is a Serbian historiographical term that is used to describe the territory that according to the dominant school of Serbian historiography in the late 19th century formed the core of the Serbian Empire in 1346–71.

The term does not refer to a defined region but over time in the late 19th century and the first decade of the 20th century it came to include the regions of Raška, Kosovo and Metohija and much of modern North Macedonia. The term Old Serbians (Serbian: Старосрбијанци, romanized: Starosrbijanci) were used as designations by Serb authors and later governments for Slavic populations from regions such as Vardar Macedonia. In modern historiography, the concept of Old Serbia as it developed in the 19th century has been criticised as a historical myth, based often on invented or tendentiously interpreted historical events.

Macedonians (Bulgarian: македонци), or Macedonian Bulgarians (Bulgarian: македонски българи), sometimes also referred to as Macedono-Bulgarians, Macedo-Bulgarians, or Bulgaro-Macedonians, are a regional, ethnographic group of ethnic Bulgarians, inhabiting or originating from the region of Macedonia. This is how the majority of Slavic-speaking population of Macedonia had been referred to by most of the national conscious minority among them and by outside observers, from the 10th until the early 20th century in a sense of a demonym at first, and later as an social class synonym mainly, and as ethnonym. Since 1913, the Macedonian Bulgarian population is largely concentrated in Pirin Macedonia but much is spread across the whole of Bulgaria and the diaspora.