A river border is easy to supply but how defenseable is it compared to mountains?
Really only defensible when the enemy is a backwater tribe, as seen with the whole reason the Dacian Kingdom was conquered because they were constantly launching raids into Roman Moesia.
Could they have fortified the mountain passes?
Thats what I've been advocating for the most part. Or just straight up building walls and gates along the medium sized ones and full on legionary camps and towns along the big ones.
Parthian/Sassanian Royal Army tried to force a crossing. Should that happen the Roman legions reinforcing the border would quite literally have to face an uphill battle.
With the usual Parthian/Sassanian reliance on their cavalry a closed battle in a mountain pass can only favor the infantry based Romans. And when it came to infantry battles the Romans nearly always win.
That and if they're fighting from a fortified position they should be the ones with a high ground as well as pre-placed artillery such as Scorpio, Ballistae,and early catapults meaning that the enemy would have to have a clever way of attacking.
You'd be a pretty good candidate for a darwin award if you tried to cross a highway on foot.
Yes but if you want people to not cross then a barrier is exactly what you're looking for.
However they are very easy to travel along, which makes patrolling, and sending field armies to repel invaders much more easy.
Yes but it's also far more easy for an enemy to choose an unpredictable spot to cross rather than having to choose to cross a handful of most likely fortified mountain passes.
Mountains are just as hard (or possibly even harder) to travel along as they are to travel across.
You don't need to travel across them, just mountain passes and valleys. It is literally impossible for an army to cross a mountain otherwise, spies perhaps but you're not going to be able to stop them anyways unless you have a 21st century Korean DMZ at you're disposal.
I did mention that the Tigris is also an option, and that would give the lion's share of Mesopotamia.
Maybe, but there would still be plenty of issues that would have to be addressed.
With the border that the Romans settled on IOTL it was a dagger pointed directly at Syria, and with the border you propose it would still pose a threat to Mesopotamia. Short of conquering the Iranian plateau this region is going to be a geographic gap, and the Romans seemed willing to accept that IOTL.
True enough I suppose.
The same goes for mountains too, and Mesopotamia is far from being Persia's only urbanized region.
Yes but there are no large cities or towns on the other side of the eastern zagros, the closest one would be Susa and given its going to be near that gap that people have been mentioning earlier I would say there would an emphasis on that region in the defense budget.
Wat? Did the Bronze Age and ever age after the iron age not happen?
Fair enough, I should have said after the Iranian plateau had actually begun to Urbanize and develop until they could afford to mass armies and form states. But when you take that into account from then to the age of Firearms the Mesopotamian region has usually been under the influence of an Iranian based power with short blips here and again and almost never independent(i.e. under Rome, Arabia, ect) with the largest being under the Caliphates I believe.
Given that Rome's Danube and Rhine frontiers held for centuries, I'd say that they're quite defendable.
They're defendable against poor backwater tribes who have more to loose than gain from fighting(kind of when the Migration period happened the Rivers didn't stop tribes from crossing though the civil wars and such probably didn't help with that) and serves mostly as an arbitrary line only to serve to slow down any invading force long enough to catch up to and defeat.
Against an organized kingdom or Empire using a river border does not work as well, OTL the whole reason for the conquest of Dacia was because they kept crossing the Danube and raiding Moesia.
they could also allow prefabricated scorpions and larger ballistas to be quickly moved via boat which could then act as weapons platforms on arrival.
Maybe. But in the Mountain passes you could have those right exactly where the enemy will be coming from.
.... fair point. Do we have any examples of note to draw from there?
The closest I can think of is the Assyrian empire.