Danube in the context of Moravian Gate

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 to the north. It covers a territory of 110,994 square kilometres (42,855 sq mi) and is 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 Burgas, Plovdiv, and Varna.

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 permanently invaded the Balkans in the late 7th century. They established the First Bulgarian Empire, victoriously 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. Under the rule of the Krum's dynasty, the country rose to the status of a mighty empire and great power. 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 Black Sea is a marginal sea lying between Europe and Asia, east of the Balkans, south of the East European Plain, west of the Caucasus, and north of Anatolia. It is bounded by Bulgaria, Georgia, Romania, Russia, Turkey, and Ukraine. The Black Sea is supplied by major rivers, principally the Danube, Dnieper and Dniester. Consequently, while six countries have a coastline on the sea, its drainage basin includes parts of 24 countries in Europe.

The Black Sea, not including the Sea of Azov, covers 436,400 km (168,500 sq mi), has a maximum depth of 2,212 m (7,257 ft), and a volume of 547,000 km (131,000 cu mi).Most of its coasts ascend rapidly.These rises are the Pontic Mountains to the south, bar the southwest-facing peninsulas, the Caucasus Mountains to the east, and the Crimean Mountains to the mid-north.In the west, the coast is generally small floodplains below foothills such as the Strandzha; Cape Emine, a dwindling of the east end of the Balkan Mountains; and the Dobruja Plateau considerably farther north. The longest east–west extent is about 1,175 km (730 mi). Important cities along the coast include (clockwise from the Bosporus) the northern suburbs of Istanbul, Burgas, Varna, Constanța, Odesa, Yalta, Kerch, Yevpatoria, Sevastopol, Novorossiysk, Sochi, Poti, Batumi, Rize, Trabzon, Ordu, Simferopol, Samsun and Zonguldak.

Macedonia (Latin: Macedonia; Ancient Greek: Μακεδονία) was a province of ancient Rome, encompassing the territory of the former Antigonid Kingdom of Macedonia, which had been conquered by the Roman Republic in 168 BC at the conclusion of the Third Macedonian War. The province was created in 146 BC, after the Roman general Quintus Caecilius Metellus defeated Andriscus of Macedon, the last self-styled King of Macedonia in the Fourth Macedonian War. The province incorporated the former Kingdom of Macedonia with the addition of Epirus, Thessaly, and parts of Illyria, Paeonia and Thrace.

During the Republican period, the province was of great military significance, as the main bulwark protecting the Aegean region from attacks from the north. The Via Egnatia, which crossed the province from west to east was of great strategic importance, providing the main overland link between Rome and its domains in the Eastern Mediterranean. In this period, campaigns against the Dardani and Scordisci to the north and the Thracians to the east were nearly constant. By the first century BC, the province notionally extended to the Danube.

The First Bulgarian Empire (Church Slavonic: блъгарьско цѣсарьствиѥ, romanized: blŭgarĭsko tsěsarǐstvije; Bulgarian: Първо българско царство) was a medieval state that existed in Southeastern Europe between the 7th and 11th centuries AD. It was founded in 680–681 after part of the Bulgars, led by Asparuh, moved south to the northeastern Balkans. There they secured Byzantine recognition of their right to settle south of the Danube by defeating – possibly with the help of local South Slavic tribes – the Byzantine army led by Constantine IV. During the 9th and 10th century, Bulgaria at the height of its power spread from the Danube Bend to the Black Sea and from the Dnieper River to the Adriatic Sea and became an important power in the region competing with the Byzantine Empire.

As the state solidified its position in the Balkans, it entered into a centuries-long interaction, sometimes friendly and sometimes hostile, with the Byzantine Empire. Bulgaria emerged as Byzantium's chief antagonist to its north, resulting in several wars. The two powers also enjoyed periods of peace and alliance, most notably during the Second Arab Siege of Constantinople, where the Bulgarian army broke the siege and destroyed the Arab army, thus preventing an Arab invasion of Southeastern Europe. Byzantium had a strong cultural influence on Bulgaria, which also led to the eventual adoption of Christianity in 864. After the disintegration of the Avar Khaganate, the country expanded its territory northwest to the Pannonian Plain. Later the Bulgarians confronted the advance of the Pechenegs and Cumans, and achieved a decisive victory over the Magyars, forcing them to establish themselves permanently in Pannonia.

The Serbian Empire (Serbian: Српско царство / Srpsko carstvo, pronounced [sr̩̂pskoː tsâːrstʋo]) was a medieval Serbian state that emerged from the Kingdom of Serbia. It was established in 1346 by Dušan the Mighty, who significantly expanded the state.

During Dušan's rule, Serbia was one of the most powerful European states and the most powerful in Southeast Europe. It was an Eastern Orthodox multi-ethnic and multi-lingual empire that stretched from the Danube in the north to the Gulf of Corinth in the south, with its capital in Skopje. Dušan also promoted the Serbian Archbishopric to the Serbian Patriarchate. In the Serbian Empire, the region of Kosovo was the most prosperous and densely populated area, serving as a key political, religious, and cultural center.

Romania is a country located at the crossroads of Central, Eastern and Southeast Europe. It borders Ukraine to the north and east, Hungary to the west, Serbia to the southwest, Bulgaria to the south, Moldova to the east, and the Black Sea to the southeast. It has a mainly continental climate, and an area of 238,397 km (92,046 sq mi) with a population of 19 million people. Romania is the twelfth-largest country in Europe and the sixth-most populous member state of the European Union. Europe's second-longest river, the Danube, empties into the Danube Delta in the southeast of the country. The Carpathian Mountains cross Romania from the north to the southwest and include Moldoveanu Peak, at an altitude of 2,544 m (8,346 ft). Bucharest is the country's capital, largest urban area, and financial centre. Other major urban areas include Cluj-Napoca, Timișoara, Iași, Constanța and Brașov.

Settlement in the territory of modern Romania began in the Lower Paleolithic, later becoming the Dacian Kingdom before Roman conquest and Romanisation. The modern Romanian state was formed in 1859 with the unification of Moldavia and Wallachia under Alexandru Ioan Cuza, becoming the Kingdom of Romania in 1881 under Carol I of Romania. Romania gained independence from the Ottoman Empire in 1877, formalised by the Treaty of Berlin. After World War I, Transylvania, Banat, Bukovina, and Bessarabia joined the Old Kingdom, forming Greater Romania, which reached its largest territorial extent. In 1940, under Axis pressure, Romania lost territories to Hungary, Bulgaria, and the Soviet Union. Following the 1944 Romanian coup d'état, Romania switched sides to join the Allies. After World War II, it regained Northern Transylvania through the Paris Peace Treaties. Under Soviet occupation, King Michael I was forced to abdicate, and Romania became a socialist republic and Warsaw Pact member. After the uniquely violent Romanian revolution in December 1989, Romania began a transition to liberal democracy and a market economy.

The Dnieper or Dnepr (/(də)ˈniːpər/ (də-)NEE-pər), also called the Dnipro. is one of the major transboundary rivers of Europe, rising in the Valdai Hills near Smolensk, Russia, before flowing through Belarus and Ukraine to the Black Sea. Approximately 2,200 km (1,400 mi) long, with a drainage basin of 504,000 square kilometres (195,000 sq mi), it is the longest river of Ukraine and Belarus and the fourth-longest river in Europe, after the Volga, Danube, and Ural rivers.

In antiquity, the river was part of the Amber Road trade routes. During the Ruin in the later 17th century, the area was contested between the Polish–Lithuanian Commonwealth and the Russian Empire, dividing what is now Ukraine into areas described by its right and left banks. During the Soviet period, the river became noted for its major hydroelectric dams and large reservoirs. The 1986 Chernobyl disaster occurred on the Pripyat River, a tributary of the Dnieper, just upstream from its confluence with the Dnieper. The Dnieper is an important navigable waterway for the economy of Ukraine and is connected by the Dnieper–Bug Canal to other waterways in Europe. During the 2022 Russian invasion of Ukraine, certain segments of the river were made part of the defensive lines between territory controlled by the Russians and the Ukrainians.

The Dobruja Plateau or Dobrogea Plateau (Romanian: Podișul Dobrogei) is a plateau in eastern Romania located in the Dobruja (Romanian: Dobrogea) region, surrounded to the north and west by the Danube and to the east by the Danube Delta and the Black Sea.

Its average altitude is around 200–300 metres, higher in the northern part. The highest point is the Țuțuiatu/Greci Peak in the Măcin Mountains, at a height of 467 m.

In classical and late antiquity, Illyria (/ɪˈlɪəriə/; Ancient Greek: Ἰλλυρία, Illyría or Ἰλλυρίς, Illyrís; Latin: Illyria, Illyricum) was a region in the western part of the Balkan Peninsula inhabited by numerous tribes of people collectively known as the Illyrians.

The Ancient Greeks initially used the term Illyris to define approximately the area of northern and central Albania down to the Aoös valley (modern Vjosa) and the Bay of Vlorë, including in most periods much of the lakeland area (Ohrid and Prespa). It corresponded to the region that neighboured Macedonia and Epirus. In Roman times the terms Illyria, Illyris, or Illyricum were extended from the territory that was roughly located in the area of the south-eastern Adriatic coast (modern Albania and Montenegro) and its hinterland, to a broader region stretching between the whole eastern Adriatic and the Danube.

Theodosius I (Ancient Greek: Θεοδόσιος Theodosios; 11 January 347 – 17 January 395), also known as Theodosius the Great, was Roman emperor from 379 to 395. He won two civil wars and was instrumental in establishing the Nicene Creed as the orthodox doctrine for Nicene Christianity. Theodosius was the last emperor to rule the entire Roman Empire before its administration was permanently split between the Western Roman Empire and the Eastern Roman Empire. He ended the Gothic War (376–382), but did so on terms disadvantageous to the empire, with the Goths remaining and politically autonomous within Roman territory, albeit as nominal allies.

Born in Hispania, Theodosius was the son of a high-ranking general of the same name, Theodosius the Elder, under whose guidance he rose through the ranks of the Roman army. Theodosius held independent command in Moesia in 374, where he had some success against the invading Sarmatians. Not long afterwards, he was forced into retirement, and his father was executed under obscure circumstances. Theodosius soon regained his position following a series of intrigues and executions at Emperor Gratian's court. In 379, after the eastern Roman emperor Valens was killed at the Battle of Adrianople against the Goths, Gratian appointed Theodosius as a successor with orders to take charge of the military emergency. The new emperor's resources and depleted armies were not sufficient to drive the invaders out; in 382 the Goths were allowed to settle south of the Danube as autonomous allies of the empire. In 386, Theodosius signed a treaty with the Sasanian Empire which partitioned the long-disputed Kingdom of Armenia and secured a durable peace between the two powers.

The Ural, also known as the Yaik /ˈjaɪk/, is a river flowing through Russia and Kazakhstan in the continental border between Europe and Asia. It originates in the southern Ural Mountains and discharges into the Caspian Sea. At 2,428 kilometres (1,509 mi), it is the third-longest river in Europe after the Volga and the Danube, and the 18th-longest river in Asia. The Ural is conventionally considered part of the boundary between the continents of Europe and Asia.

The Ural rises near Mount Kruglaya in the Ural Mountains, flows south parallel and west of the north-flowing Tobol, through Magnitogorsk, and around the southern end of the Urals, through Orsk where it turns west for about 300 kilometres (190 mi), to Orenburg, where the river Sakmara joins. From Orenburg it continues west, passing into Kazakhstan, then turning south again at Oral, and meandering through a broad flat plain until it reaches the Caspian a few kilometers below Atyrau, where it forms a fine 'digitate' (tree-like) delta.

The Germanic peoples were tribal groups who lived in Northern Europe during Classical antiquity and the Early Middle Ages. In modern scholarship, they typically include not only the Roman-era Germani who lived in both Germania and parts of the Roman Empire, but also all Germanic speaking peoples from this era, irrespective of where they lived, most notably the Goths. Another term, ancient Germans, is considered problematic by many scholars because it suggests identity with present-day Germans. Although the first Roman descriptions of Germani involved tribes west of the Rhine, their homeland of Germania was portrayed as stretching east of the Rhine, to southern Scandinavia and the Vistula in the east, and to the upper Danube in the south. Other Germanic speakers, such as the Bastarnae and Goths, lived further east in what is now Moldova and Ukraine. The term Germani is generally only used to refer to historical peoples from the 1st to 4th centuries CE.

Different academic disciplines have their own definitions of what makes someone or something "Germanic". Some scholars call for the term's total abandonment as a modern construct, since lumping "Germanic peoples" together implies a common group identity for which there is little evidence. Other scholars have defended the term's continued use and argue that a common Germanic language allows one to speak of "Germanic peoples", regardless of whether these ancient and medieval peoples saw themselves as having a common identity. Scholars generally agree that it is possible to refer to Germanic languages from about 500 BCE. Archaeologists usually associate the earliest clearly identifiable Germanic speaking peoples with the Jastorf culture of the Pre-Roman Iron Age in central and northern Germany and southern Denmark from the 6th to 1st centuries BCE. This culture existed around the same time that the First Germanic Consonant Shift is theorized to have occurred, leading to recognizably Germanic languages. Germanic languages expanded south, east, and west, coming into contact with Celtic, Iranic, Baltic, and Slavic peoples before they were noted by the Romans.

Iranian peoples, or Iranic peoples, are a diverse ethnolinguistic group who are identified chiefly by their native usage of any of the Iranian languages, which are a branch of the Indo-Iranian languages within the Indo-European language family.

The Proto-Iranians are believed to have emerged as a separate branch of the Indo-Iranians in Central Asia around the mid-2nd millennium BC. At their peak of expansion in the mid-1st millennium BC, the territory of the Iranian peoples stretched across the entire Eurasian Steppe; from the Danubian Plains in the west to the Ordos Plateau in the east and the Iranian Plateau in the south.

The Goths were a Germanic people who played a major role in the fall of the Western Roman Empire and the emergence of medieval Europe. They were first mentioned by Graeco-Roman authors in the 3rd century AD, living north of the Danube in what is now Ukraine, Moldova, and Romania. From here they conducted raids into Roman territory, and large numbers of them joined the Roman military. These early Goths lived in the regions where archaeologists find the Chernyakhov culture, which flourished throughout this region during the 3rd and 4th centuries.

In the late 4th century, the lands of the Goths in present-day Ukraine were overwhelmed by a significant westward movement of Alans and Huns from the east. Large numbers of Goths subsequently began to concentrate near the Roman border at the Lower Danube, seeking refuge within the Roman Empire. Their entry into the Empire resulted in violence, and Goth-led forces inflicted a devastating defeat upon the Romans at the Battle of Adrianople in 378. Roman forces regained a level of control but many Goths and other eastern peoples were quickly settled in and near the empire. One of these groups, initially led by their king Alaric I, sacked the city of Rome in 410, they were the precursors of the Visigoths. The successors of the Visigoths eventually established a Visigothic Kingdom in Spain at Toledo. Meanwhile, Goths under Hunnic rule gained their independence in the 5th century, most importantly the Ostrogoths. Under their king Theodoric the Great, these Goths established the Ostrogothic Kingdom in Italy at Ravenna.

The Raeti (/ˈraɪtiː/ RY-tee; spelling variants: Rhaeti, Rheti or Rhaetii) were a confederation of Alpine tribes, whose language and culture were related to those of the Etruscans. Before the Roman conquest, they inhabited present-day Tyrol in Austria, eastern Switzerland and the Alpine regions of northeastern Italy. After the Roman conquest, the province of Raetia was formed, which included parts of present-day Germany south of the Danube.

The etymology of the name Raeti is uncertain. The Roman province of Raetia was named after these people.

The Golden Horde, self-designated as Ulug Ulus (lit. 'Great State' in Turkic), was originally a Mongol and later Turkicized khanate established in the 13th century, originating from the northwestern part of the Mongol Empire. After the division of the Mongol Empire in 1259, it became a functionally independent khanate. It is also known as the Kipchak Khanate or the Ulus of Jochi, and replaced the earlier, less organized Cuman–Kipchak confederation.

It originally consisted of the lands bequeathed to Jochi (d. 1225). It greatly grew in size under Batu Khan, the founder of the Blue Horde. After Batu's death in 1255, his dynasty flourished for a full century, until 1359, though the intrigues of Nogai instigated a partial civil war in the late 1290s. The Horde's military power peaked during the reign of Özbeg Khan (1312–1341), who adopted Islam. The territory of the Golden Horde at its peak extended from Siberia and Central Asia to parts of Eastern Europe from the Urals to the Danube in the west, and from the Black Sea to the Caspian Sea in the south, while bordering the Caucasus Mountains and the territories of the Mongol dynasty known as the Ilkhanate.

Between 28 June and 3 July 1940, the Soviet Union occupied Bessarabia and Northern Bukovina, following an ultimatum made to Romania on 26 June 1940 that threatened the use of force. Those regions, with a total area of 50,762 km (19,599 sq mi) and a population of 3,776,309 inhabitants, were incorporated into the Soviet Union. On 26 October 1940, six Romanian islands on the Chilia branch of the Danube, with an area of 23.75 km (9.17 sq mi), were also occupied by the Soviet Army.

The Soviet Union had planned to accomplish the annexation with a full-scale invasion, but the Romanian government, responding to the Soviet ultimatum delivered on 26 June, agreed to withdraw from the territories to avoid a military conflict. The use of force had been made illegal by the Conventions for the Definition of Aggression in July 1933, but from an international legal standpoint, the new status of the annexed territories was eventually based on a formal agreement through which Romania consented to the retrocession of Bessarabia and cession of Northern Bukovina. As it was not mentioned in the ultimatum, the annexation of the Hertsa region was not consented to by Romania, and the same is true of the subsequent Soviet occupation of the Danube islands. On 24 June, Nazi Germany, which had acknowledged the Soviet interest in Bessarabia in a secret protocol to the 1939 Molotov–Ribbentrop Pact, had been made aware prior to the planned ultimatum but did not inform the Romanian authorities and was unwilling to provide support. On 22 June, France, a guarantor of Romanian borders, fell to Nazi advances. This is considered to be an important factor in the Soviets' decision to issue the ultimatum. The Soviet invasion of Bukovina in 1940 violated the Molotov-Ribbentrop Pact, since it went beyond the Soviet sphere of influence that had been agreed with the Axis.