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Third Bulgarian state

The Treaty of San Stefano was signed on 3 March 1878 by Russia and the Ottoman Empire. It was to set up an autonomous Bulgarian principality spanning Moesia, Macedonia and Thrace, roughly on the territories of the Second Bulgarian Empire, and this day is now a public holiday called National Liberation Day. The other Great Powers immediately rejected the treaty out of fear that such a large country in the Balkans might threaten their interests. It was superseded by the Treaty of Berlin, signed on 13 July. It provided for a much smaller state, the Principality of Bulgaria, only comprising Moesia and the region of Sofia, and leaving large populations of ethnic Bulgarians outside the new country. This significantly contributed to Bulgaria's militaristic foreign affairs approach during the first half of the 20th century.

The Bulgarian principality won a war against Serbia and incorporated the semi-autonomous Ottoman territory of Eastern Rumelia in 1885, proclaiming itself an independent state on 5 October 1908. In the years following independence, Bulgaria increasingly militarised and was often referred to as "the Balkan Prussia ". It became involved in three consecutive conflicts between 1912 and 1918—two Balkan Wars and World War I. After a disastrous defeat in the Second Balkan War, Bulgaria again found itself fighting on the losing side as a result of its alliance with the Central Powers in World War I. Despite fielding more than a quarter of its population in a 1,200,000-strong army and achieving several decisive victories at Doiran and Monastir, the country capitulated in 1918. The war resulted in significant territorial losses and a total of 87,500 soldiers killed. More than 253,000 refugees from the lost territories immigrated to Bulgaria from 1912 to 1929, placing additional strain on the already ruined national economy.

Between 19 October 1925 and 29 October 1925, the Incident at Petrich, nicknamed "the War of the Stray Dog" occurred, which was a minor armed conflict. Greece invaded Bulgaria, after the killing of a Greek captain and sentry by Bulgarian soldiers. The conflict was settled by the League of Nations, and resulted in a Bulgarian diplomatic victory. The League ordered a ceasefire, Greek troops to withdraw from Bulgaria and Greece to pay £45,000 to Bulgaria.

The resulting political unrest led to the establishment of a royal authoritarian dictatorship by Tsar Boris III (1918–1943). Bulgaria entered World War II in 1941 as a member of the Axis but declined to participate in Operation Barbarossa and saved its Jewish population from deportation to concentration camps. The sudden death of Boris III in mid-1943 pushed the country into political turmoil as the war turned against Germany, and the communist guerrilla movement gained momentum. The government of Bogdan Filov subsequently failed to achieve peace with the Allies. Bulgaria did not comply with Soviet demands to expel German forces from its territory, resulting in a declaration of war and an invasion by the USSR in September 1944. The communist-dominated Fatherland Front took power, ended participation in the Axis and joined the Allied side until the war ended. Bulgaria suffered little war damage and the Soviet Union demanded no reparations. But all wartime territorial gains, with the notable exception of Southern Dobrudzha, were lost.

The left-wing coup d'état of 9 September 1944 led to the abolition of the monarchy and the executions of some 1,000–3,000 dissidents, war criminals, and members of the former royal elite. But it was not until 1946 that a one-party people's republic was instituted following a referendum. It fell into the Soviet sphere of influence under the leadership of Georgi Dimitrov (1946–1949), who established a repressive, rapidly industrialising Stalinist state. By the mid-1950s, standards of living rose significantly and political repression eased. The Soviet-style planned economy saw some experimental market-oriented policies emerging under Todor Zhivkov (1954–1989). Compared to wartime levels, national GDP increased five-fold and per capita GDP quadrupled by the 1980s, although severe debt spikes took place in 1960, 1977 and 1980. Zhivkov's daughter Lyudmila bolstered national pride by promoting Bulgarian heritage, culture and arts worldwide. Facing declining birth rates among the ethnic Bulgarian majority, Zhivkov's government in 1984 forced the minority ethnic Turks to adopt Slavic names in an attempt to erase their identity and assimilate them. These policies resulted in the emigration of some 300,000 ethnic Turks to Turkey.

The Communist Party was forced to give up its political monopoly on 10 November 1989 under the influence of the Revolutions of 1989. Zhivkov resigned and Bulgaria embarked on a transition to a parliamentary democracy. The first free elections in June 1990 were won by the Communist Party, now rebranded as the Bulgarian Socialist Party. A new constitution that provided for a relatively weak elected president and for a prime minister accountable to the legislature was adopted in July 1991. The new system initially failed to improve living standards or create economic growth—the average quality of life and economic performance remained lower than under communism well into the early 2000s. After 2001, economic, political and geopolitical conditions improved greatly, and Bulgaria achieved high Human Development status in 2003. It became a member of NATO in 2004 and participated in the War in Afghanistan. After several years of reforms, it joined the European Union and the single market in 2007, despite EU concerns over government corruption. Bulgaria hosted the 2018 Presidency of the Council of the European Union at the National Palace of Culture in Sofia.