As noted in more detail in the SparkNote on the Fall of the Roman Empire, beginning in the middle of the 3rd century CE, the Roman Empire faced increasing Germanic tribe infiltration and internal political chaos. Romans set up generals as emperors, who were quickly deposed by rival claimants. This pattern continued until Diocletian (r. 284-305) rose to power in 285. He and Constantine (324-337) administratively reorganized the empire, engineering an absolute monarchy. Constantine the Great patronized Christianity, particularly in his new city Constantinople, founded on the ancient site of Byzantium. Christianity became the Roman state religion under Theodosius (r. 379-95). Germanic tribal invasions also proceeded, as did battles with the Sassanids in the East. From 375, Gothic invasions, spurred by Hun marauding, began en masse. Entanglement with imperial armies resulted in increased migration into Roman heartlands as far as Iberia. The Empire underwent a certain Germanization. After the death of Theodosius, the Eastern Empire followed its own course, evolving into Hellenized Byzantium by the seventh century. Repeated sackings of Latin Rome (410, 455), contraction of food supplies, and deposition of the last Western emperor by the Odovacar (476), ended any hope of recoveringPax-Romana in the Mediterranean basin. Gaul was controlled by a shifting patchwork of tribes.
Heroic attempts of the Eastern Emperor Justinian (r. 527-565) to retake once-Roman Italy, North Africa, and parts of Gaul, were only temporarily successful, as western apathy, the tax burden the campaigns imposed, and Lombard invasions into Italy prevented any lasting gains beyond southern Italy. By 600 CE, Byzantium consisted of a sliver of North Africa, Nilotic Egypt, a few Mediterranean Islands, the southern Balkans and Thrace, as well as Anatolia and the Levant littoral. The Avar Khanate was well-established beyond the Danube, Franks occupied Germany and France, just as the Visigoths controlled all of Spain but the southern sliver, and the Angles and Saxons had moved into southern Denmark and western Britain.
The next two centuries (600-800) were instrumental for the creation of medieval civilization. Politically, Byzantium faced the explosion of the Avars as far as Thrace. Additionally, renewed Sassanid Persian offensives deprived Byzantium of the state of Eastern Anatolia as well as the Levant, the birthplace of Christianity. Heraclius (r. 610-641) ultimately defeated the Sassanids and restored the status quo, yet the exhaustion caused by the wars, the precariousness of reestablished Byzantine control in the Near East, as well as continuing theological controversies allowed the political and military arrival of Islam in the 630s to eject the Byzantines from all of the Middle East, excluding Anatolia. Wedged between Avar domination in the Balkan and Adriatic regions and Arab occupation of long-time Roman lands, 'Fortress Byzantium' began to take form as the Orthodox Christian standard bearer in opposition to Islam, with only nominal, formal concern for the West.
In the West, while Lombards and other various tribes held Italy in uneasy alliances, the three-way split of France between the Burgundians, Visigoths and Franks had been decided in favor of the latter, in the form of the Merovingian dynasty of Clovis and his sons (482-560s). Continual partitioning under descendants, dynastic infighting, and the sheer limits of seventh-century coercive force, contributed to disintegration of central control, whereby provincial counts took localized power for themselves, and Palace deputies usurped much of the power of the consistently young-dying Merovingian kings. One mayor, Pepin II, subdued his counterparts in other Merovingian lands and united the realms. His son Charles Martel, in addition to defeating the Muslims at Tours (732), extended family control further to the East. His son Pepin III dethroned the last Merovingian with church support, then was invited into Italy to curtail Lombard harassment of the Papacy. Given the title 'Protector of the Romans' by Pope Stephen II (752), the emerging dynasty cultivated ties with the Church, utilizing its spiritual authority, empowering its parish priests, and facilitating the institution of tenth-century papal states. Pepin's son Carlos Magnus (Charlemagne, r. 768-814) inaugurated the Carolingian dynasty. Based on a territorial core of modern France, his forces conquered and Catholicized the Saxons of Northern Germany, as well as the Bavarians, and increased Frankish/Catholic influence in Bohemia, Poland, and the Slavic/Czech Adriatic regions. Repeatedly on the Pope's request, his forces went south, finally subduing the Lombards and establishing political control there. Though coronated Holy Roman Emperor by the Pope in 800, Charlemagne's descendants fell to infighting, even after three kingdoms were established in the Carolingian domains in 843.
In the ninth century the Carolingian empire continued its disintegration, and Viking and Norman raids extended to inland regions of Spain, France, and Italy on a nearly yearly basis, while rising Muslim naval activity in the central Mediterranean further imperiled trade and Italian polities. Eventually, the Normans established statelets in Northwestern France, Apulia, and Sicily. In the latter two instances the Normans displaced Muslims: the first hints of Reconquista/Crusading fervor. These processes brought about a severe localization of European power, evidenced by the emergence of feudalism, based upon personal bonds of vassalage, and a manor system organizing agricultural production and rural security. Bishoprics also became prominent in providing administration, justice, and moral guidance. From the 600s onward, the Papacy expanded hierarchically, demonstrating an increased independence from Constantinople manifested in doctrinal differences and near schisms in the ninth century. Monasticism arose, energizing the Church and papacy. Beginning in the Middle East and given a European foundation by the Benedictine Code (529), successive Monastic reform movements in the ninth century and then in the tenth-eleventh century (Cluny) gave greater vigor to Church attempts to a) preserve the remnants of classical learning; b) elaborate theology; c) lessen fighting in Europe while encouraging Reconquista. In addition, as monastics became popes, the Church became able to assert increased claims to a spiritual papacy with worldly powers.
The political complexion of Europe simplifies in the second quarter of the 10th century, as post-Carolingian notable elites elevated the dukes of Franconia (Conrad and Henry I, r. 918-36) to kingship. These new rulers subjugated duchies that would not relinquish power. Otto I (936-73) was able to continue subjugation of eastern kingdom duchies, hold back and defeat the Magyars of Hungary (955), attack and further Christianize Slavs, tentatively enforce authority over north-central Italy, and be crowned emperor. His grandson Otto III was likewise crowned in 996, after causing his cousin to be crowned as Pope Gregory V. Ecclesiastical expansion continued. Still, the western Carolingian realms (France) remained wallowed in the localized chaos of feudal duchies, consenting in 987 to the election of Hugh Capet as nominal king, who ruled over a drastically curtailed realm.
Meanwhile, after enduring sieges of Constantinople by Arab Muslims (674, 680, 717), Byzantium soldiered on, evolving a tenuous means of coexistence with its neighbors to the east that entailed land tenure and a militarization of Asia Minor's civil administration. Byzantium followed an approach to Christianity, called Orthodoxy, completely independent from that sanctioned by the Papacy. The state itself managed all church matters, appointing patriarchs, and often determining 'right' doctrine, as in the Iconoclastic controversies (mid-800s). Fallen on hard times in the ninth century, Byzantium underwent a resurgence in the tenth, owing to political disunity within Abbasid Islamic world and the evolution of a more viable Byzantine system. Along with marked successes against the Bulgars under Basil II (r. 963-1025), a short-lived Byzantine advance in southern Italy accompanied the recovery of Crete and Cyprus from the Muslims (965) and progress in northeastern and southeastern Anatolia. The 1054 schism between the Catholic and Orthodox churches finalized the cultural, political, and religious split between Byzantine and Latin Europe. Left no time to find rapprochement with the western Church, by 1054 Byzantium, whose defenses had decayed from renewed neglect, faced the onslaught of Turkic tribes against its eastern borders.