If the tribes of Germans, Suevians, and Goths were repulsed from Italian soil, they had already taken a firm position on the soil of Gaul. Cæsar (De Bello Gallico) speaks of Ariovistus and the Suevians whom he met there as his most formidable enemies. They did not appear to him as forming an isolated body, detached from Germany; on the contrary, he relates that the Germans time and again forced their way into Gaul. Movements within continued, for even after Augustus we find that the Romans did not always encounter the same peoples in the same countries. This is affirmed by Procopius, Paulus Diaconus, and many others.
Let us recall here that already after the death of Nero, Civilis, who was in the service of Rome, led eight cohorts from his country into Gaul where he was defeated, but he was able to make an arrangement, thanks to which he could settle as an ally at a small distance from the borders which he had betrayed (Gibbon).
The Germans (a people composed of voluntary associations of soldiers, almost savages) being hunters rather than cultivators, were naturally obliged to change their residence; we know indeed with what rapidity the game was exhausted, which obliged those who live by it to overrun an immense extent of territory and continually to transfer their residence to other places; this is why emigration is in this case the result of the law of inertia, the people not knowing how to replace a precarious form of existence by one that is more stable. They had no towns, but real movable villages that could be compared to those of the Arabs of Africa. Like all nomad peoples and hunters, when the hope of a conquest shone before them, they abandoned their forests and, desiring to reach warmer regions, went from them with their wives and children to war. During long years their efforts were impotent, because until the time of Marcus Aurelius they were divided, precisely like the savages of America, into a great number (40) of small tribes, dispersed over an immense territory and enemies of one another; it was, consequently, the more easy to subdue them, the rather that not knowing the use of the breast-plate and but little that of iron and of cavalry, they found themselves powerless against the Roman legions, of whose tactics besides they were ignorant.
But when Rome, at the decadence, commenced to recruit its army with Germans, and when less vigilant in guarding the frontiers, she allowed German families, if not even tribes, to pass the same, she found herself in great part disarmed against an enemy who had already set foot against her, bearing her own weapons, and what is worse, who knew her treasures, her tactics, and her weaknesses. Under Tiberius even, it was known that the auxiliary soldiers constituted the principal force of the Roman armies (nihil validum in exercitibus nisi quod externum); at first equal in number to the legionary soldiers, they much exceeded them afterwards, when the citizens evaded military services, and when under Gallienus the Senators were forbidden to command the army. To all these causes can still be added secondary ones.
Before the historic invasion, emigration had taken place. "When," says Gibbon, "a cruel famine befell the Germans, they had no other resource than to send a third or a fourth part of their young men to seek their fortune elsewhere."
According to the national historians, emigration was due to the disproportion between the size of the population and the means of subsistence in the region where they dwelt (Paulus Diaconus): the Germans were very prolific. As they were not agriculturists, nothing bound them to the soil; pestilence or famine, a victory or a defeat, an oracle of the Gods, or the eloquence of a chief sufficed to attract them to the warmer countries of the south. Germany was then much colder than it is at present. (Gibbon.)
The necessity of fleeing from the domination of a victorious enemy forced the Huns towards the west; religious fanaticism drove the nomad Arabs towards the great Byzantine and Persian empires; religious terror urged the Cimbri and the Teutons to throw themselves on the Gauls and on Italy.[75] Often also the taste for wine and liquors led them to invade rich countries for these gifts of God.
[75] Revue des deux mondes. June 11, 1889. Berthollet.
According to a legend, doubted by some historians, but accepted by others, among them Cipolla, the descent of the Lombards into Italy appears to have been caused by the fact that some of their companions, after having served Narses, conveyed into their country some Italian fruits which excited their curiosity.
All this will suffice to explain the movement we are considering, which commenced slowly among the Northern peoples and afterwards became unrestrainable, and to show how the law of inertia was counteracted among them.