The topic there was "Why did Rome fail to conquer the Picts?" An interesting question, which continued: "Why did they fail while the Scots from Ireland succeeded?" (The questioner reminded us that "the Picts were a loose group of tribes while the Romans were a powerful empire".)
In amongst the robust repartee (which is common on most online forums), there was some inevitable misinformation. For example, in response to the observation that "Erm, Rome did conquer Scotland", referring to the Agricolan campaigns of AD 79-84, came the put-down: "there is debate over the legitimacy of Agricola's claims, though. Some believe it to be propaganda". Some probably do, but not generally those who have studied the subject.
Equally, there were some interesting observations, such as: "the Picts did not have enough trinkets, good land, gold, badassery to be worth it", and "It was too far, too full of smelly, hairy men, too little gold and, in general, they couldn't be bothered". All quite understandable opinions, if based on nothing more than gut reaction.
How Did Rome Conquer?
Along the way, I was struck by the following perceptive contribution:
An area needs a certain amount of development before it can be forced into an empire. You need towns, significant trade in bulk necessities like grain, etc. If all you've got is a bunch of hundreds of mostly self-sufficient villages and farmsteads, then you can march an army in and march an army out without really making much of an impact. You can burn and loot a bit, but there's not enough agricultural surplus to support a big garrison, and even if you do set up big forts in the valleys controlling the major rivers and fords, no one cares, because they're not dependent on trade anyway.Interesting ideas, which Roman scholars would do well to take on board. But more relevant, perhaps, to the wider geographical question, Why Did the Romans Fail to Conquer Scotland?, than to the specific query posed by the Taleworlds questioner.
The way that places like Scotland and Wales were historically conquered was by settlement -- some other Germanic or Celtic group invaded, settled in the villages, intermarried with the existing tribes, and became the new tribal overlords. If you try that with professional soldiers, they will no longer be professional soldiers. An invading army might be able to change the demography or the language of such areas, but can't make them answer to an imperial or provincial capital.
Why Didn't the Romans Conquer the Picts?
The problem with this question is that it lacks historical perspective. Which Romans are we talking about? And did they try to conquer the Picts?
Which Romans are we talking about? In the context of the Picts, we have stepped forward into the fourth century AD. Many would agree that, by then, the Roman army had passed its prime. But even if we cling to ideas of Roman invincibility, the army that rode north from York with Constantius Chlorus in AD 305 was a very different creature from the one that had crushed the Caledonian tribes at Mons Graupius in AD 84.
Did they try to conquer the Picts? There is every indication that, by the fourth century, Rome had grown accustomed to a British province that stopped at Hadrian's Wall. Septimius Severus may have dreamed of extending Rome's dominions in Britain in AD 210, but throughout the next hundred years, no other emperor had shared his vision. Constantius Chlorus' expedition bears all the hallmarks of a punitive raid, designed to show the flag to Rome's new, aggressive neighbours. It seems to have worked. A whole generation passed before the next recorded trouble in winter AD 342/3. And almost another before the campaigns of the 360s. And another before the campaign of AD 382. And almost another before that of AD 395.
Sadly, we lack details for any of these events. But it seems clear that these are not the actions of an aggressive empire attempting conquest. Rather, they suggest an exasperated empire slapping down an increasingly irritating neighbour.
Our lack of detailed evidence means that we view the Pictish picture through a glass darkly. But one thing is clear: the Romans did not "fail to conquer the Picts". Rather, they never attempted conquest. And in doing so, they failed to solve the problem of the Picts.