The earliest evidence of human settlement, a prehistoric walled town near Provadia in E Bulgaria, dates to the 5th millenium b.c. Ancient Thrace and Moesia, which modern Bulgaria occupies, were settled (6th cent. a.d.) by Slavic tribes. In 679-80, Bulgar tribes from the banks of the Volga (see Bulgars, Eastern) crossed the Danube, subjugated the Slavs, and settled permanently in the territory of Bulgaria. The language and culture remained Slavic, and by the 9th cent. the Bulgars had fully merged with the Slavs. The first Bulgarian empire (681-1018), established by Khan Asparuhk, or Isperikh (ruled 680-701), and his successor, Terrel (ruled 701-718), soon emerged as a significant Balkan power and a threat to Byzantium. In 809 Khan Krum (ruled 803-814) captured Sofia from the Byzantines, defeated (811) Emperor Nicephorus I, besieged Constantinople, and withdrew only after obtaining yearly tribute.

In the 9th cent. Bulgaria became the arena of political and cultural rivalry between Constantinople and Rome. In 865, Boris I adopted Christianity, and in 870 Constantinople recognized the independence of the Bulgarian church. Bulgaria received Byzantine culture through the Slavic literary language developed by St. Cyril and St. Methodius in Moravia and brought to the Balkans by their disciples. The first Bulgarian empire reached its height under Simeon I (893-927), who took the title of czar. After his death the country was rent by the heresy of the Bogomils.

Bulgaria crumbled under the attacks of a reinvigorated Byzantium in the 10th cent., and in 1018 it was annexed by Emperor Basil II. Byzantine domination was weakened by the invasions of the Pechenegs and Cumans and by internal disorders at Constantinople. The second Bulgarian empire (1186-1396) rose in 1186 when Ivan Asen (Ivan I) was crowned czar at Veliko Tŭrnovo. His son, Kaloyan, crowned in 1204 with the approval of the pope, defeated (1205) Emperor Baldwin I of Constantinople. The height of Bulgar power was reached under Ivan II (Ivan Asen), whose rule (1218-1241) extended over nearly the whole Balkan Peninsula except Greece. His successors could not maintain his empire.

In 1330, Macedonian Bulgaria was conquered by Serbia. After the battles of Kosovo Field (1389) and Nikopol (1396) Bulgaria was absorbed into the Ottoman Empire. Turkish rule was often oppressive, and rebellions were frequent. By recognizing the authority of the Orthodox Eastern Church in Constantinople over all Christians in their empire, the Turks undermined the basis of Bulgarian culture. A determined effort was made to destroy Bulgarian Christianity and the Bulgarian language. The role of the Phanariots (see Phanar) was particularly resented.

Although the administration (1864-69) of Midhat Pasha made Bulgaria briefly a model province, by then Bulgarian nationalism was strong. The Mount Athos monastery had continued to use Bulgarian; there, in 1762, a monk had written a history, the first modern literary work in Bulgarian. Bulgarian schools were allowed to open in 1835. In 1870 the Bulgarian Church was reestablished. In 1876 a rebellion, led by Stefan Stambulov, broke out. The subsequent Turkish reprisals (famous as the "Bulgarian atrocities") provided a reason for the Russians to liberate (1877-78) their neighbors (see Russo-Turkish Wars).

The Treaty of San Stefano created a large autonomous Bulgaria within the Ottoman Empire—a Bulgaria that Russia expected to dominate. In order to avert the expansion of Russian influence in the Balkans, a European congress was called to revise the treaty (see Berlin, Congress of). By the new terms Bulgaria was reduced to the territory between the Danube and the Balkans. Present-day southern Bulgaria—then called Eastern Rumelia—became a separate autonomous province, and Macedonia remained under direct Turkish rule. Alexander (Alexander of Battenberg), first prince of Bulgaria, annexed Eastern Rumelia in 1885 and repulsed a consequent Serbian attack.

Alexander's successor, Prince Ferdinand of Saxe-Coburg-Gotha, profiting from the revolution of the Young Turks in the Ottoman Empire in 1908, proclaimed Bulgaria independent with himself as czar. Bulgaria was victorious against Turkey in the first (1911-12) of the Balkan Wars, but claims to Macedonia involved it in the Second Balkan War with its former allies Greece and Serbia, and it was soon defeated. By the Treaty of Bucharest (1913), Bulgaria lost S Dobruja and a large part of Macedonia.

The Macedonian issue was largely responsible for the entry in 1915 of Bulgaria into World War I on the side of Germany and Austria-Hungary. There was much domestic opposition to the war, and when Bulgaria's military position crumbled, Ferdinand fled and Boris III succeeded (1918). In the peace (see Neuilly, Treaty of) Bulgaria was forced to pay reparations and lost its outlet to the Aegean Sea to Greece and some territory to the former Yugoslavia; S Dobruja was confirmed in Romanian possession.

The Agrarian party cabinet established (1919) by Stambuliski held power until overthrown (1923) in a bloody coup. An era of political confusion ensued, dominated by the violent activities of an irredentist Macedonian terrorist group. The world economic crisis of 1929 had a disastrous impact on impoverished Bulgaria as markets for agricultural exports shrunk. In 1934, Kimon Georgiev became premier with the help of the army and ended constitutional government, but he was ousted in 1935 by Boris III, who established his personal dictatorship.

In World War II, Bulgaria saw an alliance with Germany as an opportunity to satisfy its territorial claims. In 1940, Germany forced Romania to restore to Bulgaria S Dobruja. In 1941, Bulgaria occupied parts of Yugoslavia and Greece (including Macedonia), and declared war on Great Britain and the United States—but not the Soviet Union, because the populace was pro-Russian. The child Simeon II succeeded when Boris died mysteriously (1943). In 1944 the Soviet Union declared war on Bulgaria, and Soviet troops entered the country (Sept.). Pro-Allied political forces (Communists, Agrarians, and the pro-Soviet army officers), headed by Georgiev, seized power immediately. Bulgaria declared war on Germany, and an armistice with the USSR followed (Oct.).

After a short period of coalition rule, the Communists succeeded in taking over the government. The monarchy was abolished, and in 1946 Bulgaria was proclaimed a republic with Georgi Dimitrov as premier. The peace treaty with the Allies (1947) allowed Bulgaria to keep S Dobruja, but no gains were made in Macedonia. Dimitrov proceeded to eliminate possible opponents; Agrarian leader Nikola Petrov was executed in 1947. A new constitution was enacted, and Bulgaria became a one-party state. Industry was nationalized and farms collectivized.

Bulgaria closely followed the Soviet Union in its domestic and foreign policies; after the expulsion of Yugoslavia from the Cominform in 1948, Bulgaria sided with the USSR. Dimitrov's successor, Vulko Chervenkov, massively purged the Communist party (1950). In 1951-52, Bulgaria deported to Turkey some 160,000 citizens of Turkish origin. Relations with Greece and Turkey improved somewhat after 1954. Bulgaria joined (1949) the Council for Mutual Economic Assistance and in 1955 became a member of the Warsaw Treaty Organization and the United Nations.

In the mid-1950s the government loosened its grip somewhat. Stalinists fell from power and purge victims were rehabilitated (posthumously in some cases). In 1965 army officers and party officials unsuccessfully attempted a coup. Bulgaria aided the USSR in the invasion of Czechoslovakia in 1968. In 1971, Todor Zhivkov, who had been premier since 1962, became president. In the mid-1980s, a "Bulgarization" campaign was launched against the ethnic Turks. Turks were forced to adopt Bulgarian names, and Turkish-language broadcasts and publications were halted. In 1986, Zhivkov experimented with limited economic reforms such as a "self-management" program for industrial workers. Im mid-1989, after ethnic Turkish rights groups mounted protests in May against the government, some 370,000 ethnic Turks left for Turkey in a forced exodus, though many later returned. Zhivkov's ouster in Nov., 1989, set off a year of social and political turmoil.

In Aug., 1990, the first non-Communist political leader in 40 years, Zheliu Zhelev, was elected president. Economic reforms were introduced and a new constitution (1991) created a parliamentary democracy in the country. No party, however, was able to establish a long-term government, and major economic reforms proved difficult to enact. In 1994, the Socialist party (formerly the Communists) and its allies won a parliamentary majority at the polls, and Zhan Videnov, a Socialist, became premier early in 1995. A period of hyperinflation and economic stagnation followed, and charges of corruption were widespread.

Petar Stoyanov, of the Union of Democratic Forces, was elected president in 1996, and his party won parliamentary elections held in 1997; Ivan Kostov became premier. UN economic sanctions imposed during the 1990s on neighboring Yugoslavia (since dissolved into the nations of Serbia and Montenegro), a major trade partner, had serious negative effects on Bulgaria's economy. In the parlimentary elections of 2001, the National Movement for Simeon II (NMS), a party sponsored by the former king, captured 43% of the vote and half the seats, and Simeon became premier. In the presidential election later in the year, Socialist Georgy Parvanov won the post after a runoff, defeating the incumbent, Stoyanov. Bulgaria became a member of NATO in Mar., 2004, and a little more than a year later the country signed an accession treaty with the European Union (EU).

Parliamentary elections in June, 2005, resulted in a victory for the Socialists, but they did not win a majority and were initially unable to form a coalition, and subsequently NMS also failed to do so. In August, however, the Socialists, NMS, and the largely Turkish Movement for Rights and Freedoms agreed to form a coalition government. Socialist Sergei Stanishev became premier. President Parvanov was reelected in Oct., 2006.

On Jan. 1, 2007, Bulgaria became a member of the EU, but EU concerns over Bulgarian corruption led the EU in 2008 to suspend more than €500 million in aid to Bulgaria; roughly two fifths of that aid subsequently was denied to Bulgaria. Corruption and judicial concerns also delayed the nation's joining the EU's borderless Schengen Area. Elections in July, 2009, gave the anticorruption Citizens for European Development of Bulgaria (GERB), led by Sofia Mayor Boiko Borisov, a near majority of the seats in parliament and some 40% of the vote, and Borisov subsequently became prime minister of a minority government. Rosen Plevneliev, the GERB candidate, was elected president in Oct., 2011.