Final answer:
The strategic defense of the Roman Empire changed significantly when the Goths were allowed to settle in Roman territory, an event that contributed to the downfall of the Western Roman Empire. This, combined with other internal and external challenges, weakened Roman power and ultimately led to the Empire's disintegration, particularly in the West.
Step-by-step explanation:
The strategic defense of the Roman Empire changed significantly when c. the Goths were permitted to cross the Danube River and settle in Roman territory. This event triggered what historical retrospectives describe as a 'barbarian domino effect', leading to the disintegration of the western Roman Empire. The influx of barbarians, along with other internal and external factors, like the decline of small family farms, pressures from the Sassanids, and internal political strife, led to the steady erosion of Roman power in the West. In contrast, the Eastern Empire, later known as the Byzantine Empire, was able to maintain its borders more effectively, in part because many of the barbarian invasions were directed westward and Persia did not launch major invasions during this crucial period.
Furthermore, the Roman Empire had traditionally relied on practices such as dividing and conquering enemies, negotiating from a position of strength, establishing colonies, building well-constructed roads for military deployment, and integrating former enemies as allies and citizens. These methods were less effective against the larger, better-organized barbarian groups that challenged the later Roman Empire.