Previously I mentioned the eating from the inside out of the Western Roman empire by the Germanic tribes. There is quite the story there. For now I'll get a rough sketch in, focusing on the changing borders and the composition of Roman society.
Early Rome saw constantly expanding frontiers which were relatively open and porous, but as the empire matured and its boundaries reached their peak, and the surrounding processes of exploitation advanced deeper, their traditional enemies in the form of barbarian tribes grew more organized and unified, and began to pose a greater threat to the frontier. The borders were fortified and became less porous.
These fortified borders would begin to weaken with advancing political and economic stress brought about by military overreach and resource depletion, plague, social and cultural crises, and so on, culminating in the crisis of the third century, which accelerated profound changes in the composition of the body politic.
There is debate over whether key defensive and logistical changes in this period were deliberate or simply came about through necessity and chance, but the important part is that the growing invasions were more and more managed by relaxing border controls and fortifying key cities, garrisons, and supply centers, so that as the various invaders advanced deeper and deeper into the empire, their supply chains would be lengthened and strained and their efforts and pillage and local resupply would be frustrated, as more localized military forces would triangulate on their location like a closing net. These structural changes would precipitate the walled city state as the eventual center of gravity in medieval society, which would expand again and unify until greater empires with expanding borders were again instantiated, another interesting story itself.
For now, the invading tribes would gradually make inroads in settlement, striking deals with a weakening Roman state to coexist, offering to serve in Roman armies as part of the agreements, becoming more and more integrated in Roman society, so as to shore up the weakening state and be played off against still hostile tribes.
There was eventually a period of warfare in the late 300's AD with the Goths in particular which would prove transformative, and it is a fascinating period to pick apart and analyze in its own right.
A large group of Goths arrived on the Danube border, seeking refuge within the empire itself, as they were fleeing a Hun invasion themselves. The tides and pressures in borderlands like the steppes, within which more and more powerful nomadic tribes in the centers would put more and more pressure on and invade weaker tribes on the outer edges, putting pressure on the borders of settled societies in turn, is another fascinating dynamic to keep in mind for another time.
Anyway, it was agreed that the large Goth contingent could settle, but they were improperly settled at that, from the perspective of the Roman empire anyway. They were settled as one coherent and continuous group, as opposed to breaking them and scattering them throughout the empire and assimilating them, and allowing much of them to remain armed, for various speculated reasons.
There was a resource shortage where the large group was settled, and logistics like food supplies were botched and/or sabotaged, and the group, starving and desperate began to enter into exploitative agreements with Romans such as selling their children into slavery for dog meat. Unsurprisingly, this situation became intolerable and the Gothic contingent revolted and raged across the countryside.
Typically, the nomad peoples were not skilled or equipped for siege warfare, so they would roam and pillage the countryside while avoiding the fortified cities, circulating indefinitely and ravaging the landscape, as material and political resources were insufficient to defeat the circulating force (sound familiar?) setting resources and allies and Roman detractors free to gather strength as a marauding force, culminating in the disastrous Battle of Adrianople in which a terrible rarity occurred: a Roman emperor was killed in the course of the defeat.
A settlement would be reached and the Goths would be reintegrated into Roman society, bringing about political transformations in which the integration of barbarian tribes would advance and accelerate, plugging them deeper into civil and military life, their leaders rising in the political leadership in Rome, gathering social resources in the process, eventually sacking Rome under Alaric and gaining greater political and economic footholds as the Western half of the empire continue to fragment.
