The invasions of the Roman Empire by Germanic tribes starting c.160C.E. seems in hindsight like a sudden eruption of a brand new threat. However, the growth of these tribes spanned two centuries before they were able to successfully challenge Rome. There were three main lines of development in this regard, two of them amounting to aggressive policies on the Romans’ part. The most blatant of these was a policy of mounting continuous attacks on the Germanic tribes bordering the empire not just in order to keep them at bay, but also to keep the legions busy and to win glory for the emperors. Naturally, this provided incentive for various Germanic tribes to unite in bigger confederations. The first notable example of such a confederation was that of the Marcomanni who broke through the Danube frontier defenses in the 160s and occupied so much of Marcus Aurelius’ reign. Another Roman policy was to subsidize some tribes and turn them against others to keep them all weak and divided. In the long run, this sometimes backfired, because it gave some Germanic leaders the wealth to attract followers and build up their own power. In additions to wars, there were other points of contact between the Germanic tribes and the richer Roman civilization. Trade was the most obvious, but there were men who crossed the frontier to fight in the Roman army, as well as others who were taken as slaves by the Romans and made their way back home. Through these contacts they learned Roman military and diplomatic techniques, which helped them unite in bigger confederations and fight more effectively against the legions.
Eventually, all these factors encouraged Germanic leaders to attack Rome for a variety of reasons. One was to keep their own warriors busy. Another was to win plunder with which they could reward their warriors and attract new ones to their standards. Finally, such raids into Roman territory would hopefully keep the Romans off balance and preempt Roman attacks on their territories. By the mid second century, we can see various Germanic tribes coalescing into several large confederations and peoples: the Marcomanni, Franks, Goths, and Alemanni. Pressure kept building up between and behind these peoples, unleashing the first of their invasions, that of the Marcomanni, in the 160s. Unfortunately, that was only the beginning of centuries of such conflict.