[c] Despite some substantial unification proposals in the aftermath of the World War II, Bulgarians were the only South Slavic nation which did not join the Yugoslav federation.

While there were close ethnic, historic, linguistic and religious links between population of Yugoslavia and Bulgaria, the two states found themselves on the opposing sides in the period after the end of World War I.

[1] Relations were further strained by the Internal Macedonian Revolutionary Organization's nationalist terrorist attacks against Yugoslav rule in Macedonia from Bulgaria with government assistance.

When Bulgaria tried to suppress the IMRO in 1923, it responded by helping stage a coup d'état against Bulgarian Prime Minister Aleksandar Stamboliyski and continuing to operate as a state-within-a-state based in Petrich until 1934.

The situation escalated in response to the 1948 Tito–Stalin split after which Yugoslav relations with all Eastern Bloc countries, including Bulgaria, were either suspended or significantly strained.