Bulgaria ( (listen); Bulgarian: , romanized: Blgariya), 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 of Bulgaria 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 AD 681 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 56th 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.

Serbia ( (listen), SUR-bee-ə; Serbian: Србија, Srbija, pronounced [sř̩bija] (listen)), officially the Republic of Serbia (Serbian: Република Србија, Republika Srbija, pronounced [repǔblika sř̩bija] (listen)), is a landlocked country in Southeast Europe, at the crossroads of the Pannonian Plain and the Balkans. It shares land borders with Hungary to the north, Romania to the northeast, Bulgaria to the southeast, North Macedonia to the south, Croatia and Bosnia and Herzegovina to the west, and Montenegro to the southwest, and claiming a border with Albania through the disputed territory of Kosovo. Serbia has a population of roughly 7 million inhabitants. Its capital Belgrade is also the largest city.

Continuously inhabited since the Paleolithic Age, the territory of modern-day Serbia faced Slavic migrations in the 6th century, establishing several regional states in the early Middle Ages at times recognised as tributaries to the Byzantine, Frankish and Hungarian kingdoms. The Serbian Kingdom obtained recognition by the Holy See and Constantinople in 1217, reaching its territorial apex in 1346 as Serbian Empire. By the mid-16th century, the Ottomans annexed the entirety of modern-day Serbia; their rule was at times interrupted by the Habsburg Empire, which began expanding towards Central Serbia from the end of the 17th century while maintaining a foothold in Vojvodina. In the early 19th century, the Serbian Revolution established the nation-state as the region's first constitutional monarchy, which subsequently expanded its territory. Following casualties in World War I, and the subsequent unification of the former Habsburg crownland of Vojvodina with Serbia, the country co-founded Yugoslavia with other South Slavic nations, which would exist in various political formations until the Yugoslav Wars of the 1990s. During the breakup of Yugoslavia, Serbia formed a union with Montenegro, which was peacefully dissolved in 2006, restoring Serbia's independence as a sovereign state for the first time since 1918. In 2008, representatives of the Assembly of Kosovo unilaterally declared independence, with mixed responses from the international community while Serbia continues to claim it as part of its own sovereign territory.

Serbia is an upper-middle income economy, ranked 64th in the Human Development Index domain. It is a unitary parliamentary constitutional republic, member of the UN, CoE, OSCE, PfP, BSEC, CEFTA, and is acceding to the WTO. Since 2014, the country has been negotiating its EU accession, with the aim of joining the European Union by 2025. Serbia formally adheres to the policy of military neutrality. The country provides universal health care and free primary and secondary education to its citizens.