After the Treaty of Neuilly (1919), the Macedonian Question was kept alive owing to the conflicting aims of Bulgaria and Yugoslavia regarding the geographical region of Macedonia.
The Slav population of the Yugoslav south held onto their Bulgarian identity, despite Belgrade's tough Serbianization policy. Bulgaria, whose rulers were greatly influenced by IMRO, refused to accept the territorial status quo in the south Balkans and sought expansion into Serb-Macedonia and an outlet to the Aegean.
Indeed, IMRO launched attacks against Yugoslav and, to a lesser extent, Greek territories. IMRO was based in the region of Pirin (Bulgarian-Macedonia) and was manned mainly by Bulgarian-Macedonians from Serbia and Greece, who were motivated by intense nationalist revanchism. At the same time, the Communist International (Comintern) made decisions on the Macedonian Question unfavorable to both Greece and Yugoslavia.