Bulgaria, officially the Republic of Bulgaria, is a country in Southeast Europe. It is situated on the eastern portion of the Balkans directly south of the Danube river and west of the Black Sea. Bulgaria is bordered by Greece and Turkey to the south, Serbia and North Macedonia to the west, and Romania across the Danube river to the north. It covers a territory of 110,994 square kilometres (42,855 sq mi), making it the tenth largest within the European Union and the sixteenth-largest country in Europe by area. Sofia is the nation's capital and largest city; other major cities include Plovdiv, Varna, and Burgas.
One of the earliest societies in the lands of modern-day Bulgaria was the Karanovo culture (6,500 BC). In the 6th to 3rd century BC, the region was a battleground for ancient Thracians, Persians, Celts, and Macedonians; stability came when the Roman Empire conquered the region in AD 45. After the Roman state splintered, tribal invasions in the region resumed. Around the 6th century, these territories were settled by the early Slavs. The Bulgars, led by Asparuh, attacked from the lands of Old Great Bulgaria and then permanently invaded the Balkans in the late 7th century. They established the First Bulgarian Empire, recognised by treaty in 681 AD by the Byzantine Empire. It dominated most of the Balkans and significantly influenced Slavic cultures by developing the Cyrillic script. The First Bulgarian Empire lasted until the early 11th century, when Byzantine emperor Basil II conquered and dismantled it. A successful Bulgarian revolt in 1185 established a Second Bulgarian Empire, which reached its apex under Ivan Asen II (1218–1241). After numerous exhausting wars and feudal strife, the empire disintegrated and in 1396 fell under Ottoman rule for nearly five centuries.
The Russo-Turkish War of 1877–78 resulted in the formation of the third and current Bulgarian state, which declared independence from the Ottoman Empire in 1908. Many ethnic Bulgarians were left outside the new nation's borders, which stoked irredentist sentiments that led to several conflicts with its neighbours and alliances with Germany in both world wars. In 1946, Bulgaria came under the Soviet-led Eastern Bloc and became a socialist state. The ruling Communist Party gave up its monopoly on power after the revolutions of 1989 and allowed multiparty elections. Bulgaria then transitioned into a democracy.
Since adopting a democratic constitution in 1991, Bulgaria has been a parliamentary republic composed of 28 provinces, with a high degree of political, administrative, and economic centralisation. A developed country, its advanced, high-income economy is part of the European single market and is largely based on services, followed by manufacturing, mining, and agriculture. Bulgaria has been influenced by its role as a transit country for natural gas and oil pipelines, as well as its strategic location on the Black Sea. Bulgaria is a member state of the European Union, including the Schengen Area and the eurozone, NATO, and various regional suborganizations such as the Bucharest Nine.
The name Bulgaria is derived from the Bulgars, a tribe of Turkic origin that founded the First Bulgarian Empire. Their name is not completely understood and is difficult to trace it back earlier than the 4th century AD; however, it is possibly derived from the Proto-Turkic word bulģha ("to mix", "shake", "stir") and its derivative bulgak ("revolt", "disorder"). The meaning may be further extended to "rebel", "incite" or "produce a state of disorder", and so, in the derivative, the "disturbers". Tribal groups in Inner Asia with phonologically close names were frequently described in similar terms, as the Buluoji, a component of the "Five Barbarian" groups, which during the 4th century were portrayed as both: a "mixed race" and "troublemakers".
Neanderthal remains dating to around 150,000 years ago, or the Middle Paleolithic, are some of the earliest traces of human activity in the lands of modern Bulgaria. Remains of Homo sapiens dated c. 47,000 years BP found in the Bacho Kiro cave indicate the earliest arrival of modern humans in Europe. The Karanovo culture arose c. 6,500 BC and was one of several Neolithic societies in the region that thrived on agriculture. The invention of goldworking is credited to the Varna culture, which produced the oldest golden jewellery in the world with an approximate age of over 6,000 years. This treasure has contributed to the understanding of social hierarchy and stratification in Europe's earliest societies.
Some time before the 12th century BC, the Thracians, one of the three primary ancestral groups of modern Bulgarians, appeared on the Balkan Peninsula. The Thracians were tribal and stateless, but nevertheless excelled in metallurgy and gave the Greeks the Orphean and Dionysian cults. Thracian unity was catalysed by the Persian invasions of present-day Bulgarian territories that began in the 6th century BC. Following the defeat of the second Persian invasion in 479 BC, most of the Thracian tribes united under king Teres to form the Odrysian kingdom. It was weakened and vassalised by Philip II of Macedon in 341 BC, attacked by Celts in the 3rd century, and finally became a province of the Roman Empire in AD 45.
Roman governance over the entire Balkan Peninsula was established by the end of the 1st century AD. Christianity began spreading in the region around the 4th century; within this current, Gothic bishop Ulfilas created the Gothic Bible—the first Germanic language book—in what is today northern Bulgaria around 381. The Byzantines inherited the Balkan region following the fall of Rome in 476, but their prolonged military campaigns against Persia left these territories vulnerable to barbarian incursions. This allowed the Slavs to enter the Balkan Peninsula and gradually turn its interior into a country of the South Slavs, who lived under a democracy. The partially Hellenised, Romanised, and Gothicised Thracians were subsequently assimilated by the newcomers.
Not long after the Slavic incursion, the region was once again invaded, this time by the Bulgars under Khan Asparukh. Their horde was a remnant of Old Great Bulgaria, an extinct tribal confederacy situated north of the Black Sea in what is now Ukraine and southern Russia. Asparukh attacked Byzantine territories in Moesia and conquered the Slavic tribes there in 680. A peace treaty with the Byzantine Empire was signed in 681, marking the foundation of the First Bulgarian Empire. The Bulgars formed a close-knit ruling minority caste.
Succeeding rulers strengthened the Bulgarian state throughout the 8th and 9th centuries. Krum introduced a written code of law and checked a major Byzantine invasion at the Battle of Pliska, where Byzantine emperor Nicephorus I was killed. Boris I abolished Bulgar paganism in favour of Eastern Orthodox Christianity in 864. The conversion was followed by a Byzantine recognition of the Bulgarian church and the adoption of the Cyrillic alphabet, developed in the capital Preslav. The common language, religion and script strengthened central authority and gradually fused the Slavs and Bulgars into a unified people speaking a single Slavic language. A golden age began during the 34-year rule of Simeon the Great, who oversaw the largest territorial expansion of the state. Literature produced in Old Bulgarian soon spread north throughout Eastern Europe, the language itself becoming a lingua franca of the region. These political, cultural and religious advances turned Bulgaria into one of the great medieval powers in Europe at that time, alongside the Byzantine Empire and the Frankish realm.
After Simeon's death in 927, the empire was weakened by wars with Magyars and Pechenegs and the spread of Bogomil heresy. Simeon's successor Peter I faced revolts by two of his three brothers, but both were quelled. His reign stabilised the empire through a favourable peace treaty with the Byzantines, reinforced with a marriage between Peter and Byzantine emperor Romanos' granddaughter. The agreement ushered in a period of 40 years of peaceful relations between the two powers and domestic prosperity. This lasted until 965, when war with Byzantium broke out once more; Peter suffered a stroke and retired to a monastery, dying in 969.