Because I spent so long working on the later Roman Empire and its political disintegration, I’m deeply wary of making too explicit a comparison between it and the present-day United States. It’s too easy to make an overdetermined, square-peg-into-round-hole type of argument simply because it’s available and I know it well.
But one area where the comparison makes a great deal of sense lies in the role of the frontier.
There are a lot of different ways of understanding the end of the Roman Empire in the west: barbarian invasions, internal dissension, economic collapse, some combination of the above, or a gradual slip into imperial senescence. The one that’s always made the most sense to me focuses on the imperial periphery.
The Roman Empire’s frontier zones were a space dominated by the Roman army, not just as a military force but also as a cultural and economic institution. When the people living beyond the frontiers - barbarians - interacted with the Roman Empire, they were really interacting with its army. Sometimes they fought it, sometimes they supplied it with food and supplies, and most often, they joined it. The result was a distinctive shade of frontier culture focused on the Roman military, but with a healthy dose of “barbarian” - wearing trousers, using Germanic words, and so on - mixed in. This culture encompassed both sides of the border, creating a zone of intense interaction stretching well into both the barbarian lands and the Roman Empire.
In the later stages of the Roman Empire’s existence in the west, it’s often hard to tell the difference between a force of rampaging barbarians and a Roman field army. Both drew their recruits primarily from people living beyond the frontier. They used the same kinds of swords and wore the same kinds of helmet: Even the famous Sutton Hoo helmet from 6th-century England is just a Roman cavalry helmet (a Spangenhelm) with a cool-looking mustachioed face mask added. Roman soldiers spoke a variety of camp Latin that was generously spiced with Germanic words. Plenty of barbarian raiders had served time in the Roman military; it’s not hard to imagine that some barbarian recruits into the Roman army had probably raided Roman territory at some point before they joined up. Even Roman soldiers recruited inside the empire’s boundaries were often descended from recently settled barbarian groups.
The upshot of all this is that rather than seeing a series of barbarian invasions that brought foreign invaders into the Roman heartlands, we should instead think of what happened as the transposition of frontier culture from the periphery to the imperial core. We can’t really draw a line between the “barbarian” and Roman military, because there wasn’t a firm distinction; the two bled into one another, and it’s easier to think of this as a militarized and ethnically distinct frontier culture. This culture, and people who had been brought up with it and molded by it, was what moved, not a distinct series of barbarian ethnic groups who were unfamiliar with Roman ways and practices.