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,[1] following the First World War.
According to the 2022 Serbian census, two municipalities, Bosilegrad and Dimitrovgrad, are populated primarily by ethnic Bulgarians.
The Internal Western Outland Revolutionary Organization (Bulgarian: Вътрешна западнопокрайска революционна организация, romanized: Vătrešna zapadnopokrajska revoljucionna organizacija), IWORO, was a Bulgarian separatist organization active in the Western Outlands between 1921 and 1941 in the Kingdom of Serbs, Croats and Slovenes (until 1929), and then its successor, the Kingdom of Yugoslavia.
[2] The organization was established in 1921 on the basis of several detachments created straight after the cession of the Western Outlands to Serbia in 1920.
The period of armed conflict started in 1922 and the organisation carried out numerous assaults on the Tzaribrod–Belgrade railway, on bridges, Yugoslav garrisons and barracks until 1941 when the region was occupied by Bulgarian troops.