The period saw a continuation of trends begun during late classical antiquity, including population decline, especially in urban centres, a decline of trade, and increased immigration. The period has been labelled the "Dark Ages", a characterization highlighting the relative scarcity of literary and cultural output from this time, especially in Northwestern Europe. The term "Dark Ages" is inappropriate when applied to the Iberian peninsula, since during the Caliphate of Córdoba and Taifas periods Spanish culture, learning, arts, and science flourished as nowhere else in Europe, and Córdoba was the largest city in the world. However, the Eastern Roman Empire, or Byzantine Empire, continued to survive, though in the 7th century the Islamic caliphates conquered swathes of formerly Roman territory.
Many of these trends were reversed later in the period. In 800 the title of emperor was revived in Western Europe by Charlemagne, whose Carolingian Empire greatly affected later European social structure and history. Europe experienced a return to systematic agriculture in the form of the feudal system, which introduced such innovations as three-field planting and the heavy plough. Barbarian migration stabilized in much of Europe, although Northern Europe was greatly affected by the Viking expansion.
Starting in the 2nd century, various indicators of Roman civilization began to decline, including urbanization, seaborne commerce, and population. Archaeologists have identified only 40 per cent as many Mediterranean shipwrecks from the 3rd century as from the first. Estimates of the population of the Roman Empire during the period from 150 to 400 suggest a fall from 65 million to 50 million, a decline of more than 20 per cent. Some scholars have connected this de-population to the Dark Ages Cold Period (300–700), when a decrease in global temperatures impaired agricultural yields.
Early in the 3rd century Germanic peoples migrated south from Scandinavia and reached the Black Sea, creating formidable confederations which opposed the local Sarmatians. In Dacia (present-day Romania) and on the steppes north of the Black Sea the Goths, a Germanic people, established at least two kingdoms: Therving and Greuthung.
The arrival of the Huns in 372–375 ended the history of these kingdoms. The Huns, a confederation of central Asian tribes, founded an empire. They had mastered the difficult art of shooting composite recurve bows from horseback. The Goths sought refuge in Roman territory (376), agreeing to enter the Empire as unarmed settlers. However many bribed the Danube border-guards into allowing them to bring their weapons.
The discipline and organization of a Roman legion made it a superb fighting unit. The Romans preferred infantry to cavalry because infantry could be trained to retain the formation in combat, while cavalry tended to scatter when faced with opposition. While a barbarian army could be raised and inspired by the promise of plunder, the legions required a central government and taxation to pay for salaries, constant training, equipment, and food. The decline in agricultural and economic activity reduced the empire's taxable income and thus its ability to maintain a professional army to defend itself from external threats.
In the Gothic War (376–382), the Goths revolted and confronted the main Roman army in the Battle of Adrianople (378). By this time, the distinction in the Roman army between Roman regulars and barbarian auxiliaries had broken down, and the Roman army comprised mainly barbarians and soldiers recruited for a single campaign. The general decline in discipline also led to the use of smaller shields and lighter weaponry. Not wanting to share the glory, Eastern Emperor Valens ordered an attack on the Therving infantry under Fritigern without waiting for Western Emperor Gratian, who was on the way with reinforcements. While the Romans were fully engaged, the Greuthung cavalry arrived. Only one-third of the Roman army managed to escape. This represented the most shattering defeat that the Romans had suffered since the Battle of Cannae (216 BCE), according to the Roman military writer Ammianus Marcellinus. The core army of the Eastern Roman Empire was destroyed, Valens was killed, and the Goths were freed to lay waste to the Balkans, including the armories along the Danube. As Edward Gibbon comments, "The Romans, who so coolly and so concisely mention the acts of justice which were exercised by the legions, reserve their compassion and their eloquence for their own sufferings, when the provinces were invaded and desolated by the arms of the successful Barbarians."