Response to Zod:
I have read most of your thread - it's fun stuff, which I liked the way I liked the Ameriwank thread, but it IS a wank. Real people and real countries have real failures. IMHO, it got utterly hopeless in the technology. The United States, with contemporary research tech, couldn't get that far from where the Empire started that fast; there are especially huge dependencies to the Bessemer process. And universities and research funding systems and the stocks needed for venture capital hadn't been invented yet.
More to the point, real monarchies never have strong tech, and big ones are even worse. In every case where democracy's been coexistent with monarchy, monarchies've been outpaced. Constantinople fell to cannon somebody else built (another monarchy, but a hungrier one). Back when China was a monarchy, despite repeated turnovers of dynasty, its record once unified was to fall hopelessly behind the rest of the world by banning anything that could force elites to have to adapt. Every contemporary monarchy in which the King is executive, unbalanced by rival insitutions like parliaments and courts, is hopelessly behind the long-standing democracies in heartbreaking numbers of things, even the Jordan that tries to keep itself open. Why would your TL be different?f
Problems for monarchies and other unfree societies include: A culture of seeing Kings as religiously above mortals and mortal reality (EVERY monarchy's been religiously backed). Officials are afraid that the successful can bootstrap themselves to be more powerful than they are. And, not least is that a better widget or theory is implicitly both a critcism of seniors in power because it makes their ideas look obsolete, and often need adapting to. Though some modern monarchies have freedom of speech laws and provisions, not one abides by it.
Look how much crankiness the Internet's caused in FREE societies. In an unfree society, I'd have to live in fear for my Internet work, and the Internet would've been introduced in at best a slow way that didn't threaten media or political elites.
Thus, even China, though its society's two notches better-run than it was as a monarchy, has no Googles. It has a handful of big high-tech players, distinguished more for their guanxi (contacts) than leading-edge tech. There's nobody like our Google or Digital Equipment Corporation who've created a market empire around new tech.
Wozza:
I hope I can interest you in reading my answer to Zod, first.
What economic stasis? It was a prosperous, thriving economy until overwhelmed by invaders. Your view is entirely anachronistic. The Roman Empire was incredibly lightly governed by modern standards. Cities were self-governing, and the Imperial government was irrelevant to most peoples' daily lives.
REALLY!?!
It had more and more economic regulation over time, including regulation limiting kids to doing the same things their parents did. Such dynamism, yes.
And, from the first random page in Marcellinus, p121: "Long afterwards Cornelius Gallus, who was Egypt's procurator in the reign of Octavian, ruined the city [of Thebes] by peculation in a massive scale." Second random page,373: "Remigius, who, as I related earlier, had supported count Romanus in his depredations in the provinces, was succeeded as master of the offices by..."
Of course, the Late Republic (though mostly just in the Late Republic) was bad that way, too, with the need to raise more and more money to run for consul, but that's hardly the picture ofa thriving economy. At least even Late Republican Rome had a thriving Rome, at least before Marius, but Imperial Rome and Constantinople suffered from being the locations of the Courts. NOTHING big happened without bribery, or somebody investigating you; both were even worse than our DC.
Elite politics were little different - people killed each other. The Republic ended because generals couldn't be given a triumph without taking over, the system was not what you called "balanced." It experiences decades of internal turmoil and repeatedly reverted to one man rule.
... for most of a century, yes, after that fool Marius put in a fatal bug. After 4 centuries of pretty good rule for those times.
You are talking about time periods in which there is not even a clear distinction between internal and external. ....
I'm not understanding what you're getting at here.
Basileus Giorgios:
Yeah, I was a bit oblique, though I DID respond to your late-Empire-conquering point. I'll try to be more direct, like Dell.
I felt like your list of true threats vs minor fish had an opportunistic flavor to it - every threat the Republic faced was minor, and every threat the Empire had trouble with happened to be major.
My explanation's different - actions have consequences, and I think my long list of differences in approach, along with the Empire's falling behind its smaller and hus more competitive neighbors in military innovation, are likelier explanations for the differences in the changes in the borders.