Southeast Europe (820 – 963 CE): Bulgar Ascendancy, Byzantine Restoration, and the Slavic Christianization

Geographic and Environmental Context

Southeast Europe in this age stretched from Thrace and the Lower Danube through the Haemus/Balkan and Carpathian corridors to the Adriatic coasts and Greek peninsulas.

• Core lowlands: the Lower Danube–Wallachian plains, Dobruja steppe–coast, and the Thracian plain around Adrianople (Edirne).

• Mountain gates: Shipka and other Balkan passes; Carpathian gates into Transylvania; the Via Militaris (Belgrade–Niš–Sofia–Adrianople–Constantinople) and Via Egnatia (Dyrrachium–Thessaloniki) bound interior and sea.

• Sea nodes: Black Sea ports of Dobruja and Adriatic city-ports (Split, Zadar, Trogir, Ragusa) tied agrarian interiors to Mediterranean and Pontic trade.
  A cool–temperate regime trended toward the Medieval Warm Period after c. 950, lengthening growing seasons in the Danube–Thrace lowlands while Danube flood pulses and steppe drought cycles continued to shape transport and incursions.

Societies and Political Developments

Eastern Balkans: Bulgaria and the Byzantine–Danubian Frontier

• First Bulgarian Empire (Pliska → Preslav):

  • Boris I (r. 852–889) adopted Christianity (864/865), aligning diplomatically with Constantinople while pursuing church autonomy.

  • The Cyril–Methodius mission (863) and the return of their disciples founded the Ohrid and Preslav Literary Schools, standardizing Glagolitic → Cyrillic and enabling Slavic liturgy and administration.

  • Simeon I “the Great” (r. 893–927) reached Bulgaria’s apogee—victory at Achelous (917), sieges of Constantinople, imperial titulature, and a brilliant court culture at Preslav.

  • Peter I (r. 927–969) secured peace and imperial recognition (927) but faced rising Magyar (after c. 895) and Pecheneg pressures; the Bogomil movement (c. 930s) signaled social–religious tensions within the new Christian order.

• Byzantine Thrace and Macedonia themes: Under the Macedonian dynasty (Basil I, Leo VI, Constantine VII), the empire restored forts along the Via Militaris, reasserted logistics to Adrianople, and balanced war and diplomacy with Bulgaria. In 963, Nikephoros II Phokas’s accession strengthened the professional army, though Balkan policy still prioritized positional defense and treaty management.

Danubian North and Marchlands

• South of the Carpathians and across the Lower Danube, Slavic and Romance communities—early loci of Romanian ethnogenesis—moved between Bulgarian suzerainty and Byzantine diplomacy.

• The Magyars settled the Carpathian Basin (c. 895–907), ending Great Moravia and redrawing the northwest frontier; Pechenegs entered the Lower Danube by the mid-10th century, altering raid/tribute dynamics.

• Along the Sava–Danube confluences, local principalities leveraged tolls and crossings to navigate Bulgar, Byzantine, and later Magyar power.

Western Balkans and Greece: Adriatic Cities, Croatia, and Serbian Lands

• Byzantine Greece (outside Thrace): recovery of earlier Sklaviniai; strengthening of the Themes of Hellas and Peloponnēsos; town networks and fiscal-military administration revived under Basil I and Leo VI; Constantine VII codified provincial governance. The age culminated spiritually with St. Athanasios founding the Great Lavra (963) on Mount Athos, inaugurating the Athonite commonwealth.

• Dalmatian cities (Split, Zadar, Trogir, Ragusa/Dubrovnik): Latin civic traditions under Byzantine suzerainty; maritime brokers between Venice, southern Italy, and the Aegean.

• Croatia: the duchy consolidated in the 9th c.; under Tomislav (trad. c. 925), a Kingdom of Croatia emerged, mediating between Byzantium (Dalmatian towns) and inland zones; Glagolitic liturgy—heir to Cyril–Methodius—took root alongside the Latin rite.

• Serbian principalities (Raška, Zahumlje, Travunija, Duklja): Vlastimirović rulers and coastal župans balanced Bulgar–Byzantine–Croatian pressures; baptism and church-building advanced unevenly from gradine hillfort centers.

• Bosnia & inland Slovenia: clustered hillfort communities under župans and counts tied variably to Croatian and Frankish spheres.

• North Macedonia & Vardar: Slavic polities faced alternating Bulgar and Byzantine influence; Thessaloniki anchored imperial control; the Cyril–Methodius afterglow radiated west via scriptoria and clergy.

Economy and Trade

• Agrarian base: wheat, barley, millet, and vines flourished across Thrace and the Danube plain; stock raising dominated Balkan and Carpathian piedmonts.

• River–sea circuits: Danube and Black Sea nodes (e.g., Drastar/Silistra, Varna) moved grain, salt (Transylvania & Dobruja), wax, honey, furs, and slaves; Byzantine silk, wine, coin and steppe horses circulated inland.

• Urban & court economies: Preslav developed scriptoria and artisan quarters (metalwork, fine ceramics); Adrianople provisioned troops and caravans on the Constantinople road.

• Adriatic trade: Via Egnatia carried Balkan grain, timber, wax from Dyrrachium to Thessaloniki; Dalmatian cities shipped to Venice/Apulia; nomismata and Italian denarii accompanied cloth, wine, ceramics, and metalware.
  Interior markets (Skopje, Niš) exchanged hides, honey, wax, and slaves for Mediterranean goods.

Subsistence and Technology

• Fortifications and military systems:
  Timber–earth ramparts were upgraded to stone in key sites such as Preslav, Drastar (Silistra), and Adrianople, while chains of fortified theme frontiers guarded Danube crossings and mountain passes.
  These defenses largely followed the alignments of the old Roman limes and the late imperial praefectura Illyricum, whose forts and settlements continued to anchor medieval strategy.
  Bulgarian forces combined heavy cavalry and infantry for field campaigns, while Byzantine armies relied on the thematic system of provincial troops and garrisons. On the steppe fringes, Magyar and Pecheneg horse archers introduced deep-raiding tactics that reshaped frontier warfare along the Danube and Thrace.

• Agro-tech: terrace vines/olives in Greece; cereal rotations in lowlands; pastoral transhumance across Dinaric/Pindus slopes; lagoon salt/fish production.

• Scripts & learning: Cyrillic consolidated after 900 at Preslav, enabling Slavic law codes, diplomacy, and liturgy; Latin in Adriatic municipalities; Greek in imperial administration.

Movement and Interaction Corridors

• Via Militaris—the Balkan spine—linked Belgrade–Niš–Sofia–Adrianople–Constantinople.

• Danube: moat and highway—boat patrols, ferries, and riparian markets bound Bulgaria, Byzantium, and steppe intermediaries.

• Carpathian passes mediated Magyar arrival and later commerce; Shipka and allied passes funneled Bulgar–Byzantine armies.

• Via Egnatia tied Dyrrachium to Thessaloniki and the capital; Adriatic sea-lanes (Venice–Dalmatia–Greece) completed the maritime arc.

Belief and Symbolism

• Orthodox rulership anchored both Byzantine and Bulgar legitimacy after Boris I’s baptism; Bulgaria’s autocephalous aspirations (archiepiscopal → patriarchal claims) framed imperial style under Simeon I.

• Bogomilism (c. 930s) voiced dualist critique and social dissent, later diffusing westward.

• Monastic engines—Preslav and the Ohrid school—produced translations, homilies, and Slavic pedagogy; in the west, Athonite monasticism (from 963) re-mapped sacred geography.

• Latin and Greek rites coexisted along the Adriatic front, while Glagolitic/Cyrillic extended Slavic Christianization inland.

Adaptation and Resilience

• Bulgar–Byzantine equilibrium—war, treaty, and tribute—stabilized borders and trade, sustaining cultural florescence despite frontier shocks.

• Danube redundancy: when steppe raids impeded land routes, riverine caravans and Black Sea shipping re-routed goods.

• Cultural integration: Slavic liturgy and Cyrillic fostered administrative capacity and cohesion across diverse communities.

• Theme logistics and kastra–gradina pairings let Byzantine and inland polities absorb defeats, preserve depth, and recover.

Long-Term Significance

By 963 CE, Southeast Europe stood on two pillars:

• A Bulgar imperial court at Preslav—Christian, literate, and expansionist—radiating Slavic law and culture;

• A Byzantine Thrace and Greek heartland—re-fortified along the Via Militaris and Via Egnatia, renewing spiritual authority with Athos at the close of the age.

Around them, Magyar settlement in the Carpathian Basin and Pecheneg movements along the Lower Danube reshaped the northern rim, while Croatia’s kingship, Serbian principalities, and Dalmatian cities consolidated the Adriatic interface.
The age fixed the region’s Orthodox Slavic identity, Cyrillic literary infrastructure, and strategic centrality between Aegean, Black Sea, and steppe—foundations that would define the medieval Balkans and the contests of the following age.

Eastern Southeast Europe (820 – 963 CE): Bulgar Ascendancy, Byzantine–Danubian Frontiers, and the Slavic Christianization

Geographic and Environmental Context

Eastern Southeast Europe includes Turkey-in-Europe (Thrace), northeastern Greece (Thrace-in-Greece), nearly all of Bulgaria (except its southwestern portion), modern-day Romania and Moldova, northeastern Serbia, northeastern Croatia, and extreme northeastern Bosnia and Herzegovina.

• Core lowlands: the Lower Danube and Wallachian plains, the Dobruja steppe–coast, and the Thracian plain around Adrianople (Edirne).

• Mountain corridors: Balkan/Haemus passes (e.g., Shipka), the Carpathian gates into Transylvania, and the Via Militaris (Singidunum–Naissus–Serdica–Adrianople) crossing the central Balkans.

• Black Sea ports (Dobruja) and Danube crossings tied inland agrarian zones to Mediterranean and Pontic trade.

Climate and Environmental Shifts

• Cool–temperate with improving stability toward the Medieval Warm Period (from c. 950), lengthening growing seasons in the Danube–Thrace lowlands.

• Flood pulses along the Danube structured transport, while steppe drought cycles influenced nomadic incursions (Magyars, Pechenegs).

Societies and Political Developments

• First Bulgarian Empire (capital Pliska, later Preslav):

  • Under Khan Boris I (r. 852–889), Bulgaria adopted Christianity (864/865), aligning diplomatically with Constantinople yet asserting ecclesiastical autonomy.

  • The missions of Saints Cyril and Methodius (863) to Great Moravia and the return of their disciples (notably Clement of Ohrid, Naum) created the Ohrid and Preslav Literary Schools, standardizing Glagolitic and then Cyrillic for Slavic worship and administration.

  • Simeon I “the Great” (r. 893–927) expanded Bulgaria to its apogee, defeating Byzantines at the Battle of Achelous (917), besieging Constantinople, and assuming imperial style; Preslav became a major Slavic Christian court.

  • Peter I (r. 927–969) secured peace and recognized imperial titulature from Byzantium (927), but internal tensions and frontier pressures (especially Magyar and later Pecheneg raids) mounted.

  • The Bogomil movement (c. 930s) emerged in Bulgaria—an ascetic Christian dualism that signaled social–religious strains within the new Christian order.

• Byzantine Thrace and the Thracian Theme:

  • The empire under the Macedonian dynasty—Basil I (867–886), Leo VI (886–912), Constantine VII Porphyrogennetos (913–959)—reorganized the Thrace and Macedonia themes for frontier defense; restoration of forts along the Via Militaris protected Adrianople and the approach to Constantinople.

  • In 963, Nikephoros II Phokas ascended, strengthening the professional army; in the Balkans, however, policy remained a mix of diplomacy and positional warfare with Bulgaria.

• Danubian North (Romania/Moldova), NE Balkans & Frontier Zones:

  • South of the Carpathians and across the Lower Danube, Slavic and Romance-speaking communities (later Romanian ethnogenesis) fell within shifting spheres of Bulgarian suzerainty and Byzantine diplomacy.

  • Steppe polities shaped the northern arc: Magyars settled the Carpathian Basin (c. 895–907), ending Great Moravia and altering power balances on the northwestern edge of this subregion; Pechenegs pressed into the Lower Danube by the mid-10th century.

• Northeastern Serbia / NE Croatia / NE Bosnia (marchlands):

  • Local Slavic principalities navigated between Bulgarian, Byzantine, and, later, Magyar pressures; control of Sava–Danube confluences was strategic for tolls and troop movement.

Economy and Trade

• Agrarian base: wheat, barley, millet, and vines in Thrace and the Lower Danube; stock raising in the Carpathian and Balkan piedmonts.

• Black Sea and Danube trade: grain, salt (from Transylvania and Dobruja), wax, honey, furs, and slaves moved through Silistra (Drastar), Varna, and other river–sea nodes; Byzantine imports (silk, wine, coins) and steppe horses circulated inland.

• Urban and court economies: Preslav developed scriptoria and artisan quarters (metalwork, ceramics); Adrianople thrived as a provisioning and garrison city on the road to Constantinople.

Subsistence and Technology

• Frontier fortifications: timber–earth ramparts upgraded to stone in key sites (Preslav, Drastar, Adrianople); praetentura lines guarded Danube crossings and mountain passes.

• Military systems: Bulgar heavy cavalry and combined arms; Byzantine thematic infantry/cavalry with fortified lines; steppe composites (Magyar, Pecheneg) emphasized mounted archery and deep raiding.

• Literacy and scriptoria: Cyrillic consolidated in Preslav after 900, enabling administration, law codes, and liturgy in Old Church Slavonic; royal and monastic patronage produced translations and homiletic collections.

Movement and Interaction Corridors

• Via Militaris (Belgrade–Niš–Sofia–Adrianople–Constantinople) was the empire’s Balkan spine.

• Danube served as both moat and highway—patrolled boat fleets, ferries, and markets linked Bulgaria, Byzantium, and steppe intermediaries.

• Carpathian passes mediated Magyar migration and later commerce; Balkan passes (e.g., Shipka) channeled Bulgarian–Byzantine armies.

Belief and Symbolism

• Orthodox Christianity anchored both Byzantine and Bulgarian rulership after Boris I’s baptism; autocephalous aspirations in Bulgaria (archiepiscopal, later patriarchal status) legitimated imperial claims under Simeon I.

• Bogomilism articulated social critique and spiritual dualism in mid-10th-century Bulgaria, later radiating to the western Balkans.

• Monasteries (Preslav, Ohrid school territories) functioned as cultural engines—scriptoria, education, and diplomacy—while rural shrines and relic cults drew pilgrimage.

Adaptation and Resilience

• Bulgar–Byzantine equilibrium: alternating wars and treaties stabilized borders and trade, allowing cultural florescence despite frontier shocks.

• Danube redundancy: when steppe raids blocked land routes, riverine caravans and Black Sea shipping re-routed goods.

• Cultural integration: adoption of Cyrillic and Slavic liturgy created administrative capacity and social cohesion across diverse Slavic populations.

• Frontier garrisons and theme logistics allowed Byzantium to absorb defeats (e.g., 917) and recover strategic positions.

Long-Term Significance

By 963 CE, Eastern Southeast Europe was a two-pillar system:

• A Bulgarian imperial court at Preslav, Christian and literate, radiating Slavic culture and law;

• A Byzantine Thrace fortified along the Via Militaris and Danube, guarding the approaches to Constantinople.
  Around them, Magyar settlement in the Carpathian Basin and Pecheneg movements on the Lower Danube reshaped the northern and eastern rims. The age cemented the region’s Orthodox Slavic identity, literary infrastructure (Cyrillic), and strategic centrality between Aegean, Black Sea, and steppe worlds—frameworks that would endure into the next age.

Boris inherits the Bulgarian throne from his father, Khan Presijan.

Bulgaria's territorial, military, and political potential have made it one of the largest states in Europe.

Bulgaria's approximate frontiers are the Dnieper River in the northeast, the Carpathian Mountains in the north, the Tisa (Tisza) River in the northwest, the Adriatic Sea in the west, and the Tomorr (Tomor), Belasica, Pirin, Rhodope, and Strandzha mountains in the south.

Many Slavic tribes had settled within the boundaries of the state, together with the proto-Bulgarians, a tribe of Turkic origin that had settled in the Balkan Peninsula at the end of the seventh century.

In view of the religious, ethnic, and language difficulties between the Slavs and the Bulgars, the introduction of a common and compulsory religion for all subjects is one of the principal preconditions for the formation of a united Bulgaria.

Pagan Bulgaria needs to join the “family of Christian states,” but the existence of two competing centers of Christianity—Rome and Constantinople—make it difficult for Boris to make his choice.

In 852, he sends emissaries to Eastern Francia to confirm the peace treaty of 845.

At the time of his accession he threatens Constantinople with an invasion, but his armies do not attack, and he receives a small area in Strandzha to the southeast.

The peace treaty is not signed, however, although both states exchange temporary delegations.

The Moravian Prince Rastislav persuades Boris I to help him against East Francia in 854.

According to some sources, some Franks bribed the Bulgarian monarch to attack Louis the German.

The Bulgarian-Slav campaign is a disaster, and Louis scores a great victory and invades Bulgaria.

At the same time, the Pannonian Croats wage a war against the Bulgarians.

Both peoples had coexisted peacefully up to that time, suggesting that the Croats had been paid by Louis to attack Bulgaria and distract Boris' attention from his alliance with Great Moravia.

Boris cannot achieve any success, and both sides exchange gifts and settle for peace.

The peace between Bulgaria and Eastern Francia is restored as a result of the military actions in 855, and Rastislav is forced to fight against Louis alone.

Negotiations have taken place between Bulgarian khan Boris and Photios on the status of the Bulgarian diocese but have not lead to the result expected by the Bulgarians.

The Greeks demand that the Bulgarian church organization should be entirely subjected to Constantinople.

A dissatisfied Boris renews his diplomatic contacts with the West.

The removal of the eastern threat and the rise of the Empire’s confidence also opens up opportunities in the west, where the Bulgarian ruler Boris has been negotiating with the Pope and Louis the German for a possible conversion of himself and his pagan people to Christianity.

This expansion of Rome's ecclesiastic influence up to Constantinople's very doorstep cannot be tolerated by the imperial government.

The victorious eastern armies are in 864 transferred to Europe and invade Bulgaria, in a demonstration of military might that persuades Boris to accept Constantinople’s missionaries instead.

Boris is baptized, taking the name Michael in honor of the emperor, thus beginning the Christianization of Bulgaria and initiating the nation's absorption into the Greek-influenced, Eastern Orthodox world.

Boris had originally intended to accept Roman Christianity, but an unsuccessful war with the empire forces him to adopt the Orthodox faith of Constantinople in 864.

At night and in secret, a Greek bishop and priests who have been sent to Pliska, the Bulgarian capital, baptize Boris (who at his baptism takes the Christian name Michael), his family, and the nobles who support his policy.

There is serious opposition by both the nobility and the common people to Boris' attempt to enforce mass baptism.

A pagan rebellion breaks out, and Boris retaliates by executing fifty-two boyars, together with their families.

Boris sends embassies to Pope Nicholas and to Louis the German in 866; the Pope’s immediate response is to send a mission to Bulgaria.

Local Slav chiefs in the area of present Romania have apparently entered into a vassal relationship with the Bulgarian tsars, who, following the conversion of Boris I to Christianity in 864, serve as religious and cultural intermediaries between Dacia and the Empire.

Rome and Constantinople compete for political and cultural influence in Bulgaria.

Orthodox missionaries, including Cyril and Methodius, lead the proselytization of Bulgaria, Serbia, and eventually Russia.

The Eastern Empire wins in 870 when Bulgaria accepts Eastern Rite (Orthodox) Christianity and an autocephalous Bulgarian Church is established.

This decision opens Bulgaria to Byzantine culture (and territorial ambitions) through the literary language devised for the Slavs by Cyril and Methodius.

Establishment of a common, official religion also permanently joins the Bulgarian and Slavic cultures.