IMRO ... the name of the terrorist organization active in Bulgaria, Greece, and Serbia between 1893 and 1935...
Bulgaria's involvement in international terrorism began in the early twentieth century when it provided sanctuary and a base of operations to the Internal Macedonian Revolutionary Organization (IMRO) for terrorist activities against Yugoslavia and Greece. In the 1920s and 1930s, IMRO became a virtual state within a state in southwestern Bulgaria, also known as Pirin Macedonia.
Reconciliation with Yugoslavia was a necessary step toward Stamboliiski's ultimate goal of a multiethnic Balkan peasant federation. Improved Yugoslav relations required a crackdown on the powerful Macedonian extremist movement. Accordingly, Stamboliiski began a two-year program of harsh suppression of IMRO in 1921; in 1923 Yugoslavia and Bulgaria agreed at the Nis Convention to cooperate in controlling extremists.
Meanwhile, in late 1922 and early 1923, Macedonian nationalists occupied Kiustendil along the Yugoslav border and attacked government figures to protest rapprochement with Yugoslavia and Greece. Stamboliiski responded with mass arrests, an accelerated campaign against IMRO terrorism, a purge of his own fragmented and notoriously corrupt party, and a new parliamentary election. These dictatorial measures united the agrarians' various opponents (IMRO, the National Alliance, army factions, and the social democrats) into a coalition led by Aleksandur Tsankov. The communists remained outside the group. Bulgaria's Western creditors would not protect a government that had rejected their reparations policy. In June 1923, Stamboliiski was brutally assassinated by IMRO agents, and the conspirators shortly took control of the entire country with only scattered and ineffectual agrarian resistance.
Disunited Macedonian factions also continued terrorist attacks from their virtually separate state at Petrich, causing alarm in Western Europe...
IMRO also had much more latitude under the Macedonian prime minister; this meant that political assassinations and terrorism continued unabated. IMRO raids into Yugoslavia ended Bulgarian rapprochement with that country, and the Macedonians demanded preferential economic treatment under Liapchev.
In the late 1920s, the Macedonian independence movement split over the ultimate goal of its activity. The supremacist faction sought incorporation of all Macedonian territory into Bulgaria, while the federalist faction (including the IMRO terrorists) sought an autonomous Macedonia that could join Bulgaria or Yugoslavia in a protective alliance if necessary. Violence between the two groups reinforced a growing public impression that the Liapchev government was unstable.
Meanwhile, the Macedonian situation in the early 1930s blocked further attempts to heal Balkan disputes. Four Balkan conferences were held to address the Macedonian problem; but Bulgaria, fearing IMRO reprisals, steadfastly refused to drop territorial demands in Macedonia or quell Macedonian terrorist activities in the region. Such activities had continued under all Bulgaria's postwar governments, but the People's Bloc was especially inept in controlling them. The situation eventually led to the Balkan Entente of 1934, by which Yugoslavia, Greece, Turkey, and Romania pledged to honor existing borders in the Balkans. For Bulgaria the isolation inflicted by this pact was a serious diplomatic setback in southeastern Europe.
A concerted drive by the Bulgarian military against IMRO permanently reduced the power of that organization, which by 1934 had exhausted most of its support in Bulgarian society. The fact that sponsorship of Balkan terrorism finally ceased to hinder Bulgarian foreign policy was the single lasting contribution of the Velchev-Georgiev government.