My simplified view of the Roman Empire is that they built a large and powerful empire with a strong ethos that spanned most of south and middle Europe. Then a few centuries in, they suddenly went full retard, opened their borders to German immigrants while the native population engaged in gay bathhouse orgies, followed by the complete collapse of their nation leaving nothing but destruction in the wake of its influence.
People in the distant future will look upon the US in the same way, you might as well swap out some names, keep the rest the same and you wouldn't be that far off.
The "strong ethos" the Romans had was one of plunder, and what it produced was admirable only if you admire lines on a map. "Opening their borders" was an attempt to preserve the empire because centuries of mismanagement had already destroyed it and made the remnant indefensible. Tacitus penned a famous description of the Empire: "To ravage, to slaughter, to usurp under false titles, they call empire; and where they make a desert, they call it peace." The fall of the Western Empire is a direct consequence of not being able to tax nor recruit soldiers from a desert.
A weakness of the strong, centralized states of antiquity was that virtually all (~90% at a minimum, and often over 100%) of state budgets were spent entirely on the military. While the state was able to keep expanding, all that was fueling it was its stealing anything and everything in the areas it conquered to pay soldiers with. This wasn't a sustainable economic foundation. The larger the empire became the larger the army needed to defend and police it; also the bigger the number of people who need to be wined and dined and bribed not to make a mad dash for the throne. Meanwhile the domestic economy wasn't good either. The Empire's conquests had the effect of centralizing wealth in the hands of a small number of elite, which caused massive depopulation in Western provinces, with the majority of the people who DID live there living in a condition similar to medieval serfs ("engaging in gay bathhouse orgies" was left to the elites who didn't give a fuck that they had immiserated the vast majority of the population), so there weren't an abundance of free citizens you could draft or hire into an army. If you're an emperor, populous German tribes keep raiding into the Gallic heartland, and there exists no spare money and no spare manpower to defend against it, you're obviously going to be pretty amenable when some of those Germans offer:
1) To serve you and fight in your army rather than loot you, and at a far lower price than what you'd have to pay Roman soldiers, or
2) "Hey, can we settle in this land no one lives in anyway, build farms and towns and develop it, and then we'll defend it from other Germans trying to loot it and prevent them from penetrating further into the province?"
The state didn't completely collapse anyway. The Western half of the empire fell at a time when the elites had jumped ship for the richer East anyway, and the city of Rome itself was a relative backwater by the time of Odoacer, a consequence of the domestic elites' mismanagement. Those same elites kept the state going until 1453. In its smallest, weakest form right before the Turks swallowed it for good, those elites were still prioritizing their own status above the preservation of their country, and Byzantium was in a state of active or near civil war for most of the years and decades before its conquest. The historical lesson here is that letting elites destroy their countries for the sake of increasing their own wealth is what destroys countries and enables their conquest by nations with greater social cohesion, or what the Arab historian Ibn Khaldun called "asabiyyah."
Mass migration in the West today is caused by the same elite disinterest in the countries they mismanage. Our elites are fairly open about why they want mass migration, which is to keep labor prices (and thus commodity prices) low. Capitalism requires not only consistent profits, but for the
rate of profit to increase, but competition tends to force the rate of profit downward in saturated markets. Paying higher wages might be good long-term for countries, but makes individual companies less profitable and competitive, so billionaires, who already live in a different world than everyone else and don't care about their countrymen, would rather flood the labor market with as many people as it takes to depress labor prices than to pay higher wages. The effect it has on their country doesn't matter to them, and they figure they'll just jump ship if conditions in their own countries get bad enough.