In the 370s, Huns sweeping in from the Central Asian steppe broke up the Ostrogothic and Visigothic federations north of the Black Sea. Dispossessed, the Visigoths — known in the sources as Tervingi and Greuthungi — petitioned the eastern emperor Valens in the summer of 376 for permission to cross the Danube and settle within the empire. Permission was granted, but Roman officials extorted and starved the migrants. Two years later, on 9 August 378, the Visigoths shattered Valens' army near Adrianople (modern Edirne) and killed the emperor on the battlefield. Rome had not suffered a defeat of that magnitude at the hands of a Germanic force in some six centuries.

The crossing of 376 was only the trigger of the wider movement known as the Völkerwanderung. On the last day of 406, Vandal, Suevic and Alan groups crossed a frozen Rhine; they ravaged Gaul, passed into Spain, then on into North Africa, founding by 439 a Vandal kingdom centred on Carthage. The Visigoths pushed west, sacked Rome in 410, and built kingdoms based at Toulouse and later Toledo. The Franks gradually took over northern Gaul; the Burgundians settled the Rhône basin; Anglo-Saxons crossed into Britain; by the end of the sixth century the Lombards held northern Italy.

The real motor of these movements was the Huns themselves, who under Attila assembled a vast steppe-and-Carpathian empire in the mid-fifth century. Attila invaded Gaul in 451 and Italy in 452; his sudden death in 453 dissolved the confederation within a few years, but the domino effect it had set in motion kept pressing Germanic peoples against the Roman frontier. For the Western Roman Empire the pressure proved unsustainable. By the time the Germanic commander Odoacer deposed the last western emperor, Romulus Augustulus, in 476, real power had already passed to federated barbarian kingdoms.

Older accounts framed the period as the 'barbarian invasions'; modern scholarship prefers the more neutral 'migration period' or Völkerwanderung. Work by historians such as Peter Heather, Walter Goffart and Guy Halsall has shown that settlement, assimilation and transformation mattered as much as destruction: the incoming peoples adopted much of Roman law, Christianity, and administrative practice. The outcome was less a collapse than a transition — from the Mediterranean unity of the classical world to the plural, kingdom-based order of medieval Europe.