The Germanic tribes important to Roman downfall originated in Scandinavia, from which they moved south around 1000 BCE. By 100 BCE they had reached the Rhine area, and about two hundred years later, the Danube Basin, both Roman borders. The western German tribes consisted of the Marcomanni, Alamanni, Franks, Angles, and Saxons, while the Eastern tribes north of the Danube consisted of the Vandals, Gepids, Ostrogoths, and Visigoths. The Alans, Burgundians, and Lombards are less easy to define.
Caesar first observed the Germanic tribes in 51 BCE, and marked them as a possible threat. German tribes were clan-based, with blood-loyalty the basis for all bonds. Living intermittently in settled forest clearings called hamlets, they engaged in mixed subsistence cultivation of crops and animals. Cultivation was rudimentary given the hard clay soil and use of implements more suited to Mediterranean areas. There were no food surpluses, so population remained small, around one million. Without much occupational specialization, they were an iron-age culture emphasizing war.
For the first century CE, they were not a real danger to Rome: 1)Poverty ensured poor armor and weapons; 2) they had limited tactics, consisting of ambushes and a mass charge; 3) Divisions into numerous small tribes meant a lack of political cooperation; 4) There was no real, continual government beyond the clan. In peacetime, tribal assemblies made up of all free men and warriors decided issues of peace and war. They would elect temporary war chiefs, whose legitimacy ended after hostilities.
The Roman historian Tacitus described the Germans again about 100 CE. After Caesar had conquered Gaul up to the Rhine, expansion space was curtailed for the nomadic tribes, causing demographic pressure on the borders. Some Germans began to come into contact with Roman civilization at border garrisons. They greatly admired the material aspects of Roman culture, such as arms, domestic wares, etc. Small numbers were accepted for service with Roman legions, and small scale German-Roman trade relations emerged involving cattle and slaves.
Gradually, changes occurred in the tribes over the next 250 years: A) Though kinship remained the primary bond, a new kind of political formation evolved: the Comitatus. Older, successful warrior chieftains took in younger aspirants, who then raided and shared the booty with each other. This arrangement produced a professional, more lethal warrior group, where bonds were now between man and lord, the latter signaling the beginning of a small aristocracy. B) At the same time, as inter-tribe conflict increased, spurred in part by the desire to partake of Roman material culture, tribes began electing fewer, longer serving war-chiefs. C) Eastern German tribes, Goths and Vandals, gradually migrated from North Poland to the Ukraine, pressuring the Danube frontier; they also settled north of the Black Sea, to the West of the Huns. D) Around 200, small tribes began to coalesce into supra-tribal groups. Southern Germans came together into the Alamanni, while middle Rhine groups incorporated into the Franks, and the North Germans coalesced as Saxons. By the 300s there was a continual belt of barbarian tribes all along the Roman limes from the North Sea to the Black Sea. E) Increasing numbers of Germans began to serve as Roman auxiliary forces just beyond the Roman borders, learning new tactics, acquiring better materials, coming to admire Roman society even more. Some even underwent a process of partial Romanization. F) Some, the Visigoths in particular, were gradually converted to Christianity from the 340s by Ulfillias, son of a captured slave. Converting to the Arian form of Christianity soon to be branded heresy, the Visigoths slowly communicated it to the Ostrogoths, Vandals, and Burgundians.
Roman-Barbarian dynamics remained normal until 375. In the mid-360s the pagan back-sliding emperor Julian the Apostate undertook a large Sassanid campaign, taking elements of the Rhine and Danube armies with him. In the 370s, Alamanni thus raided in Gaul, but were stopped by the western Emperor Valentin. In 375, Valentin died while pushing the Sarmatians back over the Danube. He was succeeded by Gratian in the West and Valens in the East.