君士坦丁大帝312年在拉布兰旗下收复罗马之后，马上于313年签署了米兰敕令宣布基督教为罗马帝国的合法宗教。此外，君士坦丁正式地将罗马帝国的首都从罗马迁到希腊城市拜占庭，并将后者改名为君士坦丁堡（君士坦丁之城）。395年狄奥多西一世宣布基督教为罗马帝国国教，同时他也是最后一个统一的罗马帝国的皇帝，从此之后，帝国一分为二：西罗马帝国以拉文那为中心，东罗马帝国（后被称为拜占庭帝国）以君士坦丁堡为中心。西罗马不断受到日耳曼部落的掠夺（见：民族大迁徙），并在476年最终陷落于赫鲁利人首领奥多亚塞之手。罗马的权威在西部完全丧失，西部省份很快变成了一群日耳曼王国的拼凑集合。但是罗马城在罗马天主教教会的监管之下，仍然是知识的中心，在西欧很大程度保留住了罗马的经典思想。同时，在君士坦丁堡的罗马皇帝查士丁尼一世成功地将所有的罗马法法典化成为《民法大全》(529-534)。在6世纪中，东罗马帝国卷入了一系列致命的冲突之中，首当其冲的是与波斯萨珊帝国（见罗马-波斯战争）的战争，随后是新兴的伊斯兰教哈里发帝国（四大哈里发和倭马亚王朝）的进攻。至650年，埃及、巴勒斯坦和叙利亚省已经落入穆斯林军队之手，倭马亚王朝随后在7世纪和8世纪分别征服了西班牙和意大利南部（见穆斯林占领时期）。
在西欧，一种新的政治结构正在形成：罗马帝国解体后的权力真空之下，当地建立起了平民和他们耕种的土地之间的等级制度。地主收取什一税，并向当地的领主效忠。什一税用于支付国家财政和战争支出。这就是封建制度，新的领主和国王不断出现，其中最伟大的是法兰克人的统治者查里曼大帝。800年，查里曼作为大片领土的征服者，被教皇利奥三世加冕为罗马人的皇帝(Imperator Romanorum)，有效地加强了他在西欧的权力地位。查里曼的王朝是一个新的西欧日耳曼罗马帝国——神圣罗马帝国开始的标志。在其国界之外，新的力量也在积聚。基辅罗斯公国在封疆扩土，大摩拉维亚在日益成长，而盎格鲁人和萨克逊人则忙于保家戍边。
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.