a- The difficulties for a barbarian leader invading the Roman Empire beyond what a typical NESer would expect?
Going by what happened in that one NES (HeadSerf's, I believe)...
Until about the mid-fourth century AD, "barbarian", i.e. non-Roman, Europe wasn't much of a threat to the Empire because of sheer numbers. There was an empire of roughly a hundred million (give or take about ten) on the one hand, and perhaps a few hundred thousand barbs at any given time trying to get in. Numbers are a huge issue; initial success may be relatively good, but as, for example, the Goths found out in the third century, you can win at Forum Terebronii, but there are several more armies waiting to stomp on you after that. The same sort of goes for technical issues: barbs don't usually get the kind of equipment that can get through, for example, the lorica segmentata or out-range Roman missile weapons.
After the mid-fourth century, European barbarians had reached a pretty high population as compared with previous times, and actually had a population that couldn't easily be resisted by military means alone. When the Sarmatians came through eastern Europe in the first century, they were disposed of relatively easily. But now that all of these former tribes had greater populations (due to the slow transmission of Roman farming techniques and agritech during the centuries between Caesar's marches across the Rhine and Adrianople), when the Huns started moving in the 370s, they started pushing a slightly more centralized and vastly larger number of barbarians towards Rome. So one of the largest things that the barbarians had going for them wasn't extant earlier.
And then, of course, we get to the actual operations themselves. Barbarian factions won't be able to conduct most sieges, especially in the early parts of the onslaught; they simply won't have the equipment to get through a good fortress. Since the Romans have granary facilities inside their walled cities into which they bring in the harvest, barbarians won't have a lot to go on in any given area, exacerbated by the fact that they're pillaging and thus are wasting much more than they consume. They'll need to keep moving, which lowers their ability to enter a given region and establish themselves in a strong, defensible, lasting position both militarily and politically (because if they try, they'll likely starve); naturally, this isn't by any stretch impossible, but the barbarians probably won't be able to carve out a territory by purely military means in nonfertile regions, then force a peace settlement. Instead, they'll have to lay waste to one area in hopes of being granted another in which to live, or attempt to ingratiate themselves with the established imperial authority by acting as foederati, like the Goths did for Constantius III.
The way it happened for Rome was that barbarians managed to get onto Roman soil and then some of them were allowed to stay there and remain a separate, nonassimilated cultural entity (unlike virtually all previous barbarian incursions) due to various reasons (the Tervingi and Greuthungi because Theodosius couldn't beat them in combat; Alaric's Goths because Wallia made a deal with the imperials that involved them beating the living bejeezus out of the Siling Vandals and the Alans; the Hasding Vandals because Aetius had to worry about the Huns and because the Empire was no longer receiving tax revenues from a large chunk of its territory - the lands given to the foederati or those ravaged during the epic invasions of the 400s and 410s; and so on and so forth). After the barbarians had a political and economic base - which in securing they also undermined that of the central imperial authority - they were able to slowly fight the Western Empire conventionally, as states normally do, and thus, combined, gain the advantage. The real killer for Western Rome was the loss of North Africa. Without that massively profitable region, removed at a stroke by Geiseric, the Empire just didn't have the cash to support an army large enough to maintain its holdings: a key problem here is that Gaul wasn't actually a huge revenue booster, because it's mostly on the wrong side of the economic dividing line from the time before the Common Era - it was unprofitable to extend the empire there, but Julius Caesar did it anyway, partly for security and partly for
gloire and partly for God knows what else. It's instructive to see how fast the West collapsed after the final, definitive failure of the last attempt to recapture North Africa from the Vandals (Anthemius' expedition from both East and West in 468): now that it was now clear that the West couldn't get its money and thus its power back, the barbarians were able to overwhelm it in a scant eight years. (Ricimer didn't help either, removing emperors willy-nilly and fighting with Leo I over who was allowed to be emperor. And ironically enough, the destruction of the Hunnic Empire after the death of Attila set loose many of their formerly allied tribes, all of whom launched straight into Western Rome...so by saving the empire in 451 and 452, Aetius accidentally initiated the last events that would help lead to the real imperial downfall.)
To sum up: barbarians will have to figure out some way of defeating the Empire such that they are
allowed to settle within its borders as a unified political entity. Even with increased population, this won't be easy; with Rome, it took the fact that Rome had to deal with another major Power (Sassanid Persia, which was an actual threat as opposed to the Parthian kingdom), along with an emperor who wanted to make sure that he got all the credit as opposed to his younger colleague and nephew and who thus charged into an ambush with false intelligence. Anyway, they have to get inside the Empire. It helps when they do it
en masse. Deprive it of one of the more or most profitable provinces for an added bonus. Then wait a few decades, create a political, economic, and military base in your new homeland, then strike out for the center. It helps, during this last bit, to have the target be pummeled by even more barbarians at the same time, creating both exogenous shock and (relatively) internal strife.
neverwonagame3 said:
b- The difficulties for a Roman commander who wants to usurp the Empire:
i- When the Emperor has just died.
It's easier at this juncture than at the other, clearly. In the earlier days of the Empire, since the army was smaller and the emperor's only forces in Italy were the Praetorian Guard (a relatively small unit, not capable of facing down a multiple-legion field army), a field commander who could rally his troops to his cause stood a very good chance of success, especially if he has the ability to paint himself as a better alternative than, say, a child-Emperor who won't actually be ruling for awhile or the court intriguers who don't have the military's interests at heart. That can be somewhat hit and miss, obviously, and counts on the emperor's chosen successor's lack of admirable qualities, assuming that the aforementioned and recently deceased emperor *has* a successor by blood or adoption. And if the field is sufficiently clear, i.e. pretty much everybody hates the successor or if there *is* no successor, ten it comes down to forming alliances amongst the various field commanders, personal military ability, and Who Can Get To Rome and Italy First. 69 is a great example of this in the early days.
The Dominate is a little different, because the frontiers aren't closely manned by field armies as previously. There's a divide between comitatenses (field army troops; the large-scale forces that actually have quality) and the limitanei (frontier troops); in addition, many comitatensian troops are part of the Army of the Emperor's Presence, an entity in both east and west, which offers an actual military advantage to whatever successor can be scrounged up or whoever takes power in the capital. At the same time, though, commanders of the other bodies of comitatensian field armies have proportionally larger power than say a legate in the Principate. What results will likely be a more
destructive conflict between usurper and "legitimate" successor (or usurper and usurper #2) and a need to win quickly for whoever happens to be the usurper, because in a war of attrition, whoever's got the heartland/most economically productive region has the advantage. That's how Albinus lost out to Septimius Severus in the 190s (along with slightly less military skill; for an instructive and epic read, look at Dio's account of the Battle of Lugdunum): he got the frontier, while Severus got Italy and the important stuff.
neverwonagame3 said:
ii- When the Emperor hasn't just died.
It's actually not much different, aside from the fact that one replaces "successor" in the previous discourse with "emperor". The main importance is that the emperor will command more support than any successor would among the field forces and thus it will be less easy for a given commander to raise an army to fight against the constituted authority. This of course goes out the window when the emperor's pulled some unpopular stunts, like Nero (whose main real error wasn't in being all weird and stuff like Suetonius would have us believe; he basically just lowered the poor's taxes too much and the rich landowners known as the Senate got annoyed, so they bribed the Praetorian Guard to kill him...this, combined with a revolt against his tax policy by the Spanish governor Galba and by Vindex in Gaul, was what sent his regime under, not anything idiotic like "fiddling while Rome burned"), and loses the support of the military. (Actually, even Nero really didn't lose because of the military: he lost because the Senate outbid him for the Praetorians' allegiance, in a bidding war that the emperor didn't even know he was in. Vindex' revolt in 67 and 68 wasn't widely supported, and the armies of Gaul and the Rhine were split between loyalists and rebels...with help from Rufus, who brought in other field troops, the loyalists won, but Nero was already being targeted by the senators in Rome...)
So key things for any Roman general to do is to be faced with either an unpopular or nonexistent regime in Rome itself, while having the ability and luck to either sway loyalists in his army to his side while eliminating those who don't agree with him. After that, one relies almost solely on military skill in defeating both the troops of the unpopular emperor as well as those of any other usurpers who want to try for the purple too; the usual difficulties involved in allying with other opponents of the regime are also in play, and diplomatic ability to get other commanders or civil authorities to listen to you as opposed to their own ambition is pretty handy.
My main point in originally highlighting the idea that revolting would be difficult is not that a rebellion against the constituted authority in Rome as opposed to other empires or political entities would be harder, but rather that in my opinion, many - not by any means all - NESes show rebellions as awfully easy things to accomplish, mostly because the national governments either make terrible decisions or, in the case of PC factions,
are allowed to make poor choices. A rebellion by itself ought to have very low chances of succeeding if the constituted authority retains three key things: 1) a loyal and effective army, 2) an adequate supply of money, 3) the nerve to see trouble through. (Of course, if the empire in question is also experiencing exogenous shock via an attempt at conquest or support of the rebellion or such, then all bets as to crushing the rebellion are off.)
...wow, I came up with a lot. Sorry to dump all that on you, considering a lot of it is stuff that you already know and/or is off-topic and unhelpful, but I ramble easily.
Were you looking for something else, and was what I just posted helpful?