Bulgaria ( ( listen); Bulgarian: България, tr. Bǎlgariya), officially the Republic of Bulgaria (Bulgarian: Република България, tr. Republika Bǎlgariya, IPA: [rɛˈpublikɐ bɐɫˈɡarijɐ]), is a country in southeastern Europe. It is bordered by Romania to the north, Serbia and 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 the earliest societies in the lands of modern-day Bulgaria was the Neolithic Karanovo culture, which dates back to 6,500 BC. In Antiquity (6th–3rd century BC), the region became a battleground for Thracians, Persians, Celts and Macedonian Greeks until it was conquered by the Roman Empire in 45 AD. The Eastern Roman, or Byzantine, Empire lost some of these territories to an invading Bulgar horde in the late 7th century. The Bulgars then founded the first unified Bulgarian state in 681 AD which dominated most of the Balkans and significantly influenced Slavic cultures by developing 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 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, Bulgaria has been a unitary parliamentary republic with a high degree of political, administrative, and economic centralisation. The urbanized population of seven million lives mainly in Sofia and the 27 provincial capital cities, but faces significant demographic decline. 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 at 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. Widespread corruption continues to be a major socio-economic issue.