君士坦丁大帝312年在拉布蘭旗下收復羅馬之後，馬上於313年簽署了米蘭敕令宣佈基督教為羅馬帝國的合法宗教。此外，君士坦丁正式地將羅馬帝國的首都從羅馬遷到希臘城市拜占庭，並將後者改名為君士坦丁堡（君士坦丁之城）。395年狄奧多西一世宣佈基督教為羅馬帝國國教，同時他也是最後一個統一的羅馬帝國的皇帝，從此之後，帝國一分為二：西羅馬帝國以拉文那為中心，東羅馬帝國（後被稱為拜占庭帝國）以君士坦丁堡為中心。西羅馬不斷受到日耳曼部落的掠奪（見：民族大遷徙），並在476年最終陷落於赫魯利人首領奧多亞塞之手。羅馬的權威在西部完全喪失，西部省份很快變成了一群日耳曼王國的拼湊集合。但是羅馬城在羅馬天主教教會的監管之下，仍然是知識的中心，在西歐很大程度保留住了羅馬的經典思想。同時，在君士坦丁堡的羅馬皇帝查士丁尼一世成功地將所有的羅馬法法典化成為《民法大全》(529-534)。在6世紀中，東羅馬帝國捲入了一係列致命的衝突之中，首當其衝的是與波斯薩珊帝國（見羅馬-波斯戰爭）的戰爭，隨後是新興的伊斯蘭教哈裏發帝國（四大哈裏發和倭馬亞王朝）的進攻。至650年，埃及、巴勒斯坦和敘利亞省已經落入穆斯林軍隊之手，倭馬亞王朝隨後在7世紀和8世紀分別徵服了西班牙和意大利南部（見穆斯林占領時期）。
在西歐，一種新的政治結構正在形成：羅馬帝國解體後的權力真空之下，當地建立起了平民和他們耕種的土地之間的等級制度。地主收取什一稅，並嚮當地的領主效忠。什一稅用於支付國傢財政和戰爭支出。這就是封建制度，新的領主和國王不斷出現，其中最偉大的是法蘭剋人的統治者查裏曼大帝。800年，查裏曼作為大片領土的徵服者，被教皇利奧三世加冕為羅馬人的皇帝(Imperator Romanorum)，有效地加強了他在西歐的權力地位。查裏曼的王朝是一個新的西歐日耳曼羅馬帝國——神聖羅馬帝國開始的標志。在其國界之外，新的力量也在積聚。基輔羅斯公國在封疆擴土，大摩拉維亞在日益成長，而盎格魯人和薩剋遜人則忙於保傢戍邊。
When Emperor Constantine had reconquered Rome under the banner of the cross in 312, he soon afterwards issued the Edict of Milan in 313, declaring the legality of Christianity in the Roman Empire. In addition, Constantine officially shifted the capital of the Roman Empire from Rome to the Greek town of Byzantium, which he renamed Constantinople ("City of Constantine"). In 395 Theodosius I, who had made Christianity the official religion of the Roman Empire, would be the last emperor to preside over a united Roman Empire, and from thenceforth, the empire would be split into two halves: the Western Roman Empire centered in Ravenna, and the Eastern Roman Empire (later to be referred to as the Byzantine Empire) centered in Constantinople. The Western Roman Empire was repeatedly attacked by marauding Germanic tribes (see: Migration Period), and in 476 finally fell to the Heruli chieftan Odoacer. Roman authority in the West completely collapsed and the western provinces soon became a patchwork of Germanic kingdoms. However, the city of Rome, under the guidance of the Roman Catholic Church, still remained a centre of learning, and did much to preserve classic Roman thought in Western Europe. In the meantime, the Roman emperor in Constantinople, Justinian I, had succeeded in codifying all Roman law into the Corpus Juris Civilis (529-534). For the duration of the 6th century, the Eastern Roman Empire was embroiled in a series of deadly conflicts, first with the Persian Sassanid Empire (see Roman-Persian Wars), followed by the onslaught of the arising Islamic Caliphate (Rashidun and Umayyad). By 650, the provinces of Egypt, Palestine and Syria were lost to the Muslim forces, followed by Hispania and southern Italy in the 7th and 8th centuries (see Muslim conquests).
In Western Europe, a political structure was emerging: in the power vacuum left in the wake of Rome's collapse, localised hierarchies were based on the bond of common people to the land on which they worked. Tithes were paid to the lord of the land, and the lord owed duties to the regional prince. The tithes were used to pay for the state and wars. This was the feudal system, in which new princes and kings arose, the greatest of which was the Frank ruler Charlemagne. In 800, Charlemagne, reinforced by his massive territorial conquests, was crowned Emperor of the Romans (Imperator Romanorum) by Pope Leo III, effectively solidifying his power in western Europe. Charlemagne's reign marked the beginning of a new Germanic Roman Empire in the west, the Holy Roman Empire. Outside his borders, new forces were gathering. The Kievan Rus' were marking out their territory, a Great Moravia was growing, while the Angles and the Saxons were securing their borders.