Treaty of Neuilly-sur-Seine in the context of "History of Bulgaria (1878–1946)"

Thracians or Thracian Bulgarians (Bulgarian: Тракийски българи or Тракийци) are a regional, ethnographic group of ethnic Bulgarians, inhabiting or native to Thrace. Today, the larger part of this population is concentrated in Northern Thrace, but much is spread across the whole of Bulgaria and the diaspora.

Until the beginning of the twentieth century the Thracian Bulgarians were scattered in the whole of Thrace, then part of the Ottoman Empire. After the persecutions during the Preobrazhenie Uprising and the ethnic cleansing, caused to the Bulgarian population in Eastern Thrace after the Second Balkan War, these people were expelled from the area. After World War I, Bulgaria was required to cede Western Thrace to Greece. A whole population of Bulgarians in Western Thrace was expelled into Bulgaria-controlled Northern Thrace. This was followed by a further population exchange between Bulgaria and Greece (under the Treaty of Neuilly-sur-Seine), which radically changed the demographics of the region toward increased ethnic homogenization within the territories each respective country ultimately was awarded. At this period the Bulgarian Communist Party was compelled by Comintern to accept the formation of a new Thracian nation on the base of this people in order to include them in a new separate Thracian state, as a part of a future Balkan Communist Federation.

The Armistice of Salonica (also known as the Armistice of Thessalonica) was the armistice signed at 10:50 p.m. on 29 September 1918 between Bulgaria and the Allied Powers at the General Headquarters of the Allied Army of the Orient in Thessaloniki. The armistice came into force at noon on 30 September 1918. The armistice would remain in effect until the conclusion of the Treaty of Neuilly-sur-Seine, the final general peace treaty, in November 1919.

The Kingdom of Bulgaria entered World War I on the side of the Central Powers on 14 October 1915, invading and occupying parts of Serbia. However, Bulgarian forces would struggle against the Vardar offensive launched by the Allies in September 1918, causing part of the Bulgarian Army to collapse, and an open mutiny of rebellious troops who proclaimed a republic at Radomir. The Bulgarian government then requested a ceasefire on 24 September. The terms included the withdrawal of Bulgarian forces in occupied areas and demobilization of most of Bulgaria's army.

The Western (Bulgarian) Outlands (Bulgarian: Западни (български) покрайнини, romanized: Zapadni (bălgarski) pokrajnini) is a term used in Bulgarian to denote several regions located in ex-Yugoslavia, today southeastern Serbia and southeastern North Macedonia, that were traditionally part of Bulgaria and which were predominantly inhabited by ethnic Bulgarians (Bulgarians in Serbia, Bulgarians in North Macedonia).

The territories in question were ceded by Bulgaria to the Kingdom of the Serbs, Croats and Slovenes in 1920 as a result of the Treaty of Neuilly, following the World War I. According to the 2022 Serbian census, two municipalities, Bosilegrad and Dimitrovgrad, are populated primarily by ethnic Bulgarians.