The Romans would just send a new army. This is the key to why the Romans were so succesful, they just wanked themselves into greatness, by their unique military-demographic complex, which caused that they just had much more soldiers than any of their enemies. Even Hannibal who was extremaly lucky, with winning all those battles, lernt that one just cannot defeat the Roman Republic. I regard as lucky not Rome, that they won 2nd Punic War, but I regard Hannibal as a lucky guy, because he managed to do this war so long and costly for Rome. Only his genius on the battle field caused that this war wasn't ended after short time.
Was that true, though? Why couldn't they defeat the Parthians and Germanics, then? Why did they spend ten whole years, including an intervention from the emperor himself, trying to subdue some unruly Cantabrians in northern Spain?
Rome was built to fight political entities that followes the "polis" model, the city-state model, or its evolution the urbanized kingdom. It did well where it found infrastructure to use and population centers to control as it could build an effective strategy to command and conquer such lands. On the other hand it had a much more difficult time in areas with a more diffuse population and less formal political structures : the Cantabrians were finally beaten because they had no place to flee, the Illirians were not savage enough anymore. The Germans on the other hand, as well as the British tribes, lacked infrastructure and political structures to target and had room to flee : they were not defeated. More efforts in Britain could have led to victory though, but it was not to be.
But the hellenic kings and empires of the Mediterranean could, too, count on their "advanced" techniques and urban centers for their advantage to face the Romans. They could be considered as just a tough fight, if not tougher, as the isolated barbarians on the borderlands of Europe at that time.
By the time of the third Macedonian war Perseus has ratter little ressources at his disposal, too few to crush the Romans in a decisive way. Rome has also had some time to recover from the second Punic and the two first Macedonian wars while Macedonia has barely had a decade to recover from its last defeat. Treasuary and manpower are depleted, whereas Rome is rich as hell and has not yet fallen into the demographic issue that would plague it for the second half of the century. You'd need antiochid support to the Macedonian to really fight for much longer against Rome and it would be hard to achieve an alliance between the two kingdoms, among other because the Seleucid have other issues at hand...
Yes but they don't wage war on Rome's territory, so they can't target the manpower and ressources of Rome
Except Pyrrhus and Hannibal did just that. The Cimbri and Teutones were also close to invading Italy once.
Yes but how do you come to Rome's land ? Hannibal did it through the Alps, at great cost, and was able to replenish his losses with tribes that have now been crushed and colonized by Roman settlers. Also if you're Macedonian you have to arrive through Illyria, whith whom you've been at war basically since Alexander the Great died... So the land road is probably closed.
Well, Eumenes of Pergamon, according to Greg Fisher, reported in 172 BCE to the Roman Senate that Perseus had "enough grain for thirty thousand soldiers, five thousand horses, and enough money to pay for ten thousand mercenaries, all for the ammount of ten years". I don't have the numbers on the Roman side to back it up, though, but the context in which Eumenes mentions it makes it seem alarming.
The first war with the Parthians was in 50s, later the Romans had their own problems, yet they had some succeses against them, when they finally settled themselves, sadly Augustus wasn't that fat of the final war against the Parthians. And now we are talking about the Middle Republic Period. The time of their greatest successes, first conquering Italy later the subjugating of the Mediterrain. First Rome was demographicaly better than any other city-state, because contrary to them, they constantly increasing their manpower, and recruitment base during that period, by estabilishing colonies both roman and latin, which dwellers didn't stop being Romans (well, in a case of latin colonies that's more complicated, but not important here, also landplots were bigger). Also giving every colonist 7 iugers of land for him, and 7 for each his sons, guaranted that they had enough income to afford a standard legionary equipment, and that people could afford many children, which was a problem in greek city states. If you read about classical and archaic Hellas, you might see how important was an issue, how many men could afford hoplite-grade armor etc. The second important thing. The Romans also used their subjugated italian former oponents manpower in their own wars. And they fought together with the Romans. With a big loyalty, even during Hannibal's adventure, most allies remained together with the Romans. The hellenistic monarchies with a totally diffrent military and political system weren't able to field such great armies, their warfare was diffrent, and required bigger amounts of money. Macedon as an ancient kingdom, populated by Macedonians was paradoxaly able to mobilize bigger chunk of population than the Seleukid Kingdom or Egypt (in this case mobilizing native egyptians - machimoi ended with a dissaster).
I think you underrate Carthage's structural strength; while Hannibal did win several impressive victories, Carthage wethered a long series of devastating defeats throughout the Punic Wars. If say the Seleucids had taken losses like the brothers Scipio's initial Spanish campaigns, losing Cissa and Dertosa in rapid succession, they probably would have folded before long. Seriously, tally up all the crushing Roman victories of the Second Punic War, and you'll find they generally outnumber Hannibal's victories. They were smashed in Spain, in Sardinia, in Sicily, in Corsica, twice in Northern Italy, then they lost three big battles on their home turf in Africa, then Hannibal of all people had to talk his city out of raising another army to battle the Romans again. I'd be really interested in seeing an AH scenario for a victorious Carthage against the Successor kingdoms of the east; to me it seems they would have had a lot of the same advantages as the Romans.
