Bulgaria ( (listen); Bulgarian: България, romanized: Bǎlgariya), officially the Republic of Bulgaria (Bulgarian: Република България, romanized: Republika Bǎlgariya, IPA: [rɛˈpublikɐ bɐɫˈɡarijɐ]), is a country in Southeast Europe. It is bordered by Romania to the north, Serbia and North Macedonia to the west, Greece and Turkey to the south, and the Black Sea to the east. The capital and largest city is Sofia; other major cities are Plovdiv, Varna and Burgas. With a territory of 110,994 square kilometres (42,855 sq mi), Bulgaria is Europe's 16th-largest country.
One of the earliest societies in the lands of modern-day Bulgaria was the Neolithic Karanovo culture, which dates back to 6,500 BC. In the 6th to 3rd century BC the region was a battleground for Thracians, Persians, Celts and ancient Macedonians; stability came when the Roman Empire conquered the region in AD 45. The Eastern Roman, or Byzantine, Empire lost some of these territories to an invading Bulgar horde in the late 7th century. The Bulgars founded the First Bulgarian Empire in AD 681, which dominated most of the Balkans and significantly influenced Slavic cultures by developing the Cyrillic script. This state 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 Second Bulgarian Empire disintegrated in 1396 and its territories fell under Ottoman rule for nearly five centuries.
The Russo-Turkish War of 1877–78 resulted in the formation of the current Third Bulgarian State. Many ethnic Bulgarian populations were left outside its borders, which led to several conflicts with its neighbours and an alliance with Germany in both world wars. In 1946 Bulgaria became a one-party socialist state and part of the Soviet-led Eastern Bloc. The ruling Communist Party gave up its monopoly on power after the revolutions of 1989 and allowed multi-party elections. Bulgaria then transitioned into a democracy and a market-based economy.
Since adopting a democratic constitution in 1991, the sovereign state has been a unitary parliamentary republic with a high degree of political, administrative, and economic centralisation. The population of seven million lives mainly in Sofia and the capital cities of the 27 provinces. The population has declined since the late 1980s.
Bulgaria is a member of the European Union, NATO, and the Council of Europe; it is a founding state of the Organization for Security and Co-operation in Europe (OSCE) and has taken a seat on the UN Security Council three times. Its market economy is part of the European Single Market and mostly relies on services, followed by industry—especially machine building and mining—and agriculture. Bulgaria is a developing country and ranks 52nd the Human Development Index, the lowest development rank in the EU alongside Romania. Widespread corruption is a major socioeconomic issue; it ranked as the most corrupt country in the European Union for 2018.