In an article at The Scotsman Alison Campsie muses over why the Romans never managed to conquer and hold Scotland:
According to David Breeze in his book Roman Scotland, Frontier Country, the Caledonians appeared to be “doughty fighters†with Roman statesman Dio crediting the “fearsome and dangerous†men with standing their ground over the two-year battle.
Others have suggested that the “guerilla tactics†of the Caledonians were the perfect assault on disciplined Roman fighters.
While I realize that the Scots would like to think they held the Romans at bay, I think there’s another more likely explanation. The Romans couldn’t grow grapes in Scotland which meant that they couldn’t continue their way of life in permanent settlements there.
The Domesday Book of 1087, William the Conqueror’s “great survey” of England and Wales, is an excellent record of the economy there at the time. All of the vineyards listed in Domesday were south of a line running from Ely in Cambridgeshire to Gloucestershire. There was a single lonely Scottish vineyard recorded in the 12th century.
Rome rewarded its generals with land in the newly conquered territories. Places where the Roman lifestyle could not be supported were just not that valuable to them. Rome’s British holdings were its northernmost expansion and the greatest likelihood is that extending farther north into Scotland was just not worthwhile.
Empires have natural physical limits. These depend upon government, military, financial, commerce, cultural, and other factors. Communication and logistics is of vital importance. It is also important that the central government have and be felt to have an immediate influence upon everyday life.
One thing to keep in mind is that the troops of the later Empire were not the same as those of the earlier versions. They were not equipped the same, and the tactics were not the same. There were a lot more auxiliaries (mercenaries more-or-less but not quite).
There are several ways to deal with an unruly native population. You can slaughter every man, woman, and child – problem solved, harsh but effective. You can enslave everybody or enslave women and children and kill men – economical, especially for a slave labor economy. You can take some of the people (young children or teenagers) hostage – potentially good or bad, could become loyal citizen or really knowledgeable rebel. You can leave women, children, and old men alone, but force the men into military service – militarily advantage, citizenship possible as added bonus. You can employ a mass migration – usually effective, especially when moved to another hostile region where new inhabitants must deal with existing hostiles.
Taking land is usually more complicated than just rushing to the capital, overthrowing the ruler, declaring victory, and acting surprised when instead of greeting you as liberators, they treat you as occupiers.
But, what the hell do I know? I am neither black nor elegant. I am not famous or a writer of fiction.