Bulgaria ( (listen); Bulgarian: , romanized: Balgariya), officially the Republic of Bulgaria, is a country in Southeast Europe. It is situated on the eastern flank of the Balkans, and 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. Bulgaria covers a territory of 110,994 square kilometres (42,855 sq mi), and is the sixteenth-largest country in Europe. Sofia is the nation's capital and largest city; other major cities are Plovdiv, Varna and Burgas.

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 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 permanently invaded the Balkans in the late 7th century. They established Danubian Bulgaria, victoriously recognised by treaty in 681 AD by the Eastern Roman 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 (12181241). After numerous exhausting wars and feudal strife, the empire disintegrated in 1396 and fell under Ottoman rule for nearly five centuries.

The Russo-Turkish War of 187778 resulted in the formation of the third and current Bulgarian state. 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 and a market-based economy. Since adopting a democratic constitution in 1991, Bulgaria has been a unitary parliamentary republic composed of 28 provinces, with a high degree of political, administrative, and economic centralisation.

Bulgaria is a developing country, with an upper-middle-income economy, ranking 68th in the Human Development Index. Its market economy is part of the European Single Market and is largely based on services, followed by industryespecially machine building and miningand agriculture. Widespread corruption is a major socioeconomic issue; Bulgaria ranked as the most corrupt country in the European Union in 2018. The country also faces a demographic crisis, with its population shrinking annually since around 1990; it currently numbers roughly seven million, down from a peak of nearly nine million in 1988. Bulgaria is a member of the European Union, NATO, and the Council of Europe; it is also a founding member of the OSCE, and has taken a seat on the United Nations Security Council three times.

Republicanism is a political ideology centered on citizenship in a state organized as a republic. Historically, it ranges from the rule of a representative minority or oligarchy to popular sovereignty. It has had different definitions and interpretations which vary significantly based on historical context and methodological approach.

Republicanism may also refer to the non-ideological scientific approach to politics and governance. As the republican thinker and second president of the United States John Adams stated in the introduction to his famous A Defense of the Constitutions of Government of the United States of America, the "science of politics is the science of social happiness" and a republic is the form of government arrived at when the science of politics is appropriately applied to the creation of a rationally designed government. Rather than being ideological, this approach focuses on applying a scientific methodology to the problems of governance through the rigorous study and application of past experience and experimentation in governance. This is the approach that may best be described to apply to republican thinkers such as Niccolò Machiavelli (as evident in his Discourses on Livy), John Adams, and James Madison.

The word "republic" derives from the Latin noun-phrase res publica (public thing), which referred to the system of government that emerged in the 6th century BCE following the expulsion of the kings from Rome by Lucius Junius Brutus and Collatinus.This form of government in the Roman state collapsed in the latter part of the 1st century BCE, giving way to what was a monarchy in form, if not in name. Republics recurred subsequently, with, for example, Renaissance Florence or early modern Britain. The concept of a republic became a powerful force in Britain's North American colonies, where it contributed to the American Revolution. In Europe, it gained enormous influence through the French Revolution and through the First French Republic of 1792–1804.