The Hellenics pretty much bled each other dry in their spate of succession wars. And the biggest of the bunch, the Seleucids, partly got eaten alive by the expanding Parthians, civil strife and secessions (I understand Pontus, one of the break-aways, damn near gave the Romans more trouble than the Seleucids...).
Originally Posted by :
Gah, the Empire was rubbish. Apart from Augustus and the five good emperors, all the rest were pretty much useless and incapable of conquering more places.
Not really. It's just that by that point the whole thing had already started reaching the "critical mass" where maintenance of what was already conquered absorbed so much resources it was very difficult, if not downright impossible, to muster the strenght for further lasting conquests. The shift from citizen-soldiers to paid professionals also imposed certain slightly surprising problems. Apparently in the Late Antiquity period the Roman Empire at its maximum size only actually had about twice the number of soldiers at its disposal as the Republic had had around the time it had dealt with Carthage and was busily expanding into Greece and the Balkans. When you consider the size and manpower base of the two phases, that ought to tell something.
Plus, after Gaul, Britain and what they managed to grab of the Diadochi before Parthia ate the rest, the Romans were actually kind of short of good places to expand into. To the north were the Germanic lands - poor, with little existing political infrastructure that could be taken over (as partly happened in the rather comparatively sophisticated Gaul and Iberia), and full of nasty barbarians with uncomfortable degree of competence in a very frustrating guerilla-style warfare. Ditto,
mutatis mutandis, for Central Europe where at least in parts there were also highly troublesome steppe peoples to contend with; I've read the Romans mainly settled to fortify the Danube line as a natural barrier againt the buggers. Beyond the Black Sea was pretty much Right Out - too many nomads, not much worth taking over anyway, seriously far away. The wild lands of northern Britain were more or less the same story; extremely far of, poor and underdeveloped, and full of uncooperative residents who knew the terrain a bit too well for comfort - remember, much later it'd take the English many centuries to finally subdue that region too and
they were using much more sophisticated military and adminstrational systems by the later phases...
Essentially, strategic and logistical dead ends.
The East had the powerful Parthians, and the geography of the Asia Minor and Central Asia to contend with. The Romans were never able to make much lasting headway on this front, but then the same went for the Parthians and their Sassanid successors.
Another dead end.
The south and southwest had deserts, and had the Romans somehow pushed the Legions across Sahara they'd most likely all have died of the malaria and other tropical diseases. The Arabian peninsula and the nearby deserts were similarly not exactly worth even trying; indeed, they made themselves quite useful as sources of mercenaries for the Rome-Parthia/Sassanid wars.
Dead end.
To the west there was the ocean. The technology to cross it would not be around for a millenia.
...
...well, duh.
Pretty much they just plain ran out of things their logistical, military and adminstrational base allowed them to take over for good. The fact that for example the mountain regions
inside and at the edges of the empire were never fully pacified and forever served as "internal frontiers", havens for rebels and bandits, didn't help any. But to be fair it would take a good long while before
any empire would be able to truly pacify such natural trouble spots.