I didn't mean that the civil wars were the primary cause of decline (although they helped) but that they serve to distinguish the Roman Empire from modern countries and weaken the possibility of comparison. My point is really that the Western Roman Empire never had a chance -- Rome started in southern Italy and expanded because its early enemies didn't recognize it as a threat, but they couldn't get away with that again, of course.

As to whether the monarchic system was the reason Rome couldn't keep the whole Empire under one roof -- well, the Byzantines lasted, but they did so while controlling much less territory (Justinian's conquests were quickly lost), so while my argument is weak, I admit, your counterexample is also. The monarchy was successful from Octavian until Commodus; after that, it is hard to name two good rulers who succeeded each other, and impossible to name three. I think it is not by accident that during this early period the Senate was still politically relevant and vaguely democratic. But I am also just strongly tempted to blame regional divisions on poor governance, although there were probably other factors, and I am hardly a scholar.

"It never had a chance" was a five century long empire. Sure it was destined to fall, but that's the nature of empire so far, which invited comparison to modern times. And your description of the early expansion somewhat works for the conquest of Italia, but after that they were considered a threat.

>I think it is not by accident that during this early period the Senate was still politically relevant and vaguely democratic.

The senate had no actual power starting with Augustus, who turned it into a ceremonial institution. The only time it did have a shred of power was a succession crisis, and it maintained that power throughout.

>But I am also just strongly tempted to blame regional divisions on poor governance, although there were probably other factors,

I don't understand what you're saying here.

The Western Roman Empire existed on its own for 83 years, not 500. It split off in 393 with the death of Theodosius and was conquered by Odoacer in 476. That's hardly longevity.

>The senate had no actual power starting with Augustus, who turned it into a ceremonial institution.

Early Emperors cared what the Senate thought, even if they didn't have to. Later ones didn't.

https://en.wikipedia.org/wiki/Roman_Senate#Senate_of_the_Rom...

>Around 300 AD, the emperor Diocletian enacted a series of constitutional reforms. In one such reform, he asserted the right of the emperor to take power without the theoretical consent of the senate, thus depriving the senate of its status as the ultimate depository of supreme power. Diocletian's reforms also ended whatever illusion had remained that the senate had independent legislative, judicial, or electoral powers. The senate did, however, retain its legislative powers over public games in Rome, and over the senatorial order.