Thank you for your comments.
Rome split the territory and was relying on vandals and others to be their military presence in many provences. Paying mercenaries to be your army is a losing proposition, loyalty and so forth.
I like that the Romans used local governors and some tribal chiefs, to serve alongside theirs and took family members hostage back to rome to indoctrinate them and serve as a warning what would happen if you don’t cooperate.
It is remarkable to me that a city could have such power for so long.
Best wishes.
I've lived/traveled to Europe many times. Besides Paris (visiting again in 3 weeks), Rome & Italy may be my favorite destinations. The presence & influence Rome left is remarkable. Yet, if you begin to really consider their core advantages, they didn't really 'invent' anything.
Rather, all the components were already in place: literature, arts, philosophy, concrete (construction/infrastructure) and metallurgy (iron ore mining & processing). What they did have was an incredible resource pool of virgin natural resources.
Greece had already deforested their peninsula; Egypt just a distance memory. So, you take a young, vibrant region gifted with surplus, and leverage a type of Spartan approach towards life that emphasized discipline and dominance, and the result was Rome.
No one could challenge them; as the republic expanded, their legions showed up in highly disciplined order, wielding the most advanced iron weapons, well fed, nourished and ready to rumble. And rumble they did - no one stood a chance, except for those nutty Vandals/Goths on the other side of the Rhine. With each victory, tribute and taxes were excised - everyone had to pay, there wasn't any escape.
At the height of their power, 120m people were under a central, unified government. It wasn't until just 100 years ago that that many people were subject to a single form of rule in the West. But the lesson Rome teaches is that one should never rest on their laurels. The question I always wonder is what if they hadn't relaxed and indulged in sensual impressions.
Their doom was sealed when the state of military technology failed to advance. Spears, arrows, swords - all unchanged for thousands of years. Rome had the best (iron) versions, true, but it didn't take long for those same Germans to get hold of the ability to produce the same types of weapons.
What many may find surprising is the entire system began to come undone earlier - around 250AD. That's when the Goths/Vandals began to make large forays across huge areas of territory, and had to be ransomed to leave. And, as you mention, a significant number decided to stay, choosing different role levels ranging from citizen to mercenary.
In terms of the US, our own turn towards feminine, emotional issues tends to obscure the very really threat that exits from unknown unknowns. My key interest is genetic research and AI - these combinations may enable another national entity to lower the boom on us while we're busy decided what gender someone is today. Technology never sleeps - it just changes stripes. One day you're king of the hill, the next day the unexpected defeat renders you a slave.