In the seventh century the Bulgars, a Central Asian people, invaded the region south of the Danube. They mixed with the Slavs, who had arrived a century earlier, and adopted their language. They established the first Bulgarian kingdom in 681. At its greatest power, this kingdom included parts of modern Greece, Turkey, Serbia and Montenegro, Macedonia, Romania, Moldova, and Ukraine. In 811 the Bulgarians laid siege to Constantinople (Istanbul), but failed to take the city.
In the ninth century King Boris I adopted Orthodox Christianity and made it the state religion. Under the influence of the Byzantine church, Bulgaria began to use the Cyrillic alphabet. The reign of Tsar Simeon (893–927) became known as Bulgaria's golden age; in this period the empire reached its greatest extent, and the arts and architecture flourished.
By the late 10th century internal conflicts had weakened the kingdom, and in 1018 Bulgaria fell under the rule of the Byzantine empire. Byzantine rulers maintained an oppressive form of feudalism and heavily taxed the entire country. When the Byzantine empire weakened in the 12th century, two brothers from Tirnovo, Ivan and Peter Asen, led the Bulgarians in a successful revolt. The second Bulgarian kingdom lasted from 1186 to 1396, a period in which there was strong revival of the arts and trade. From 1257 to 1277, however, the throne was contested, dividing the country, and by 1330 much of Bulgaria had fallen under the rule of Serbia. Later in the century the Ottoman Turks began attacking from the south, and by 1396 they had conquered all of Bulgaria.
Turkish rule was harsh and stifled both economic and cultural development. Nationalistic fervor began to revive in the 16th century. In the next 300 years there were many local uprisings against the Turks; rebel leaders often became folk heroes. A national rebellion in 1876 was put down by the Turks with terrible cruelty. Russia came to the aid of the Bulgarians and defeated the Ottoman Empire.
The Treaty of Berlin (1878), which had concluded the Russo-Turkish War, created the principality of Bulgaria and the autonomous province of Eastern Rumelia, but both were still nominally subject to the Turkish sultan. In 1885 Bulgaria annexed Eastern Rumelia. In 1908 Bulgaria proclaimed itself an independent kingdom.
Bulgaria gained much territory in the first Balkan War (1912), but lost most of it in the second Balkan War (1913). ( In World War I the kingdom was an ally of Germany and Austria-Hungary, and again it lost territory. In World War II Bulgaria sided with Germany against the Western allies, but remained neutral toward the Soviet Union. The Soviets invaded the country in 1944, and the Fatherland Front, a Communist-dominated group that had been resisting the German alliance, seized power. Elections in 1946 established a republic and Simeon II, the nine-year-old king, was removed. In 1947, the Communists disposed of all opposition and Bulgaria became a satellite of the Soviet Union. The government socialized industry and collectivized farms through a series of five-year development plans.
In the early 1960's, the government began to reform the economic system by decentralizing decision-making to encourage local initiative. After the invasion of Czechoslovakia by Soviet and Eastern European military forces in 1968, however, central control was reestablished. Under the economic plans of the 1960's and the 1970's, industrial output increased significantly and a greater emphasis was placed on consumer goods. In the 1970's Bulgaria began to expand its trade with the West, and in 1980 it opened the country to foreign investment. Meanwhile, the Bulgarian government began a campaign to forcibly assimilate the Turkish minority into Bulgaria's Slavic society.
In the late 1980's, discontent with the dictatorship of Todor Zhivkov, who had ruled Bulgaria since 1954, arose. Reformers within the Communist party ousted Zhivkov's regime in 1989. In 1990 the National Assembly abolished the party's constitutional monopoly on political power and established a transitional government. Later in 1990, Bulgaria held its first free elections since 1931, resulting in the election of a National Assembly dominated by the Socialist party (formerly known as the Communist party). The National Assembly elected Zhelyu Zhelev president—Bulgaria's first non-Communist president in more than 45 years. Because of economic difficulties, Bulgaria's Socialist government collapsed in late 1990. Shortly afterward a coalition government was established.
In 1991 the National Assembly ratified a democratic constitution. Legislative elections in 1994 resulted in a victory for the Socialist party. The popularity of the Socialists waned and in early 1997 protests and strikes occurred throughout the country. The government fell and new elections resulted in the defeat of the Socialists. Bulgaria joined NATO in 2004. Bulgaria joined the European Union (EU) in 2007.