So I'm trying to understand the logic behind the borders of the Roman Empire at its largest extent in 117 CE under Trajan. And while I think I understand the majority of it, this part I'm not quite certain of.
I marked it in purple in this image. It's a curvy section covering mostly parts of modern day Saudi Arabia.
I know it encompasses the fertile crescent but it actually encompasses parts beyond the fertile crescent as well. So I'm not 100% sure why it does or why it ends there or why it has that particular shape.
Is it just a question of desert? I wasn't sure since it does encompass some of the Arabian desert, just not all of it. So if it's only a question of desert then why would it encompass any of it? Is it something else?
If so, can anyone tell me why that might be?
So, two things here.
First, borders being hard delineations, aside from something explicitly agreed upon between polities or very obvious landmarks or terrain features such as rivers, are a rather modern concept. The lands 'owned' by a polity in the past were largely based around either explicit landmarks or features that were owned, and population centers (and their hinterlands, usually). This doesn't really reflect well, often, with the modern basis of drawing maps since they aren't clear borders or even necessarily territory. This is why, in many maps of ancient or classical polities, they often have round or 'fuzzy' borders - because they didn't really have borders as we think of.
Secondly, that map appears to show what is the peak of Roman direct influence in that area, immediately after Trajan's creation of Provincia Mesopotamia in 116 but prior to Hadrian returning it to Parthia in 117.
So, that border is going to be fuzzy because that's the Arabian Desert. There aren't as many major settlements, especially then, and especially where the Romans hold absolute authority. Their authority basically ends where they are willing and able to impose it. They don't have a written 'hard' border - they control cities and settlements, and in some cases up to landmarks, but when one part of your territory 'ends' as a large desert, that border is fluid (though that's going to be accurate for any border that doesn't have some sort of hard or fixed delineation). In the case of the Arabian Desert, this was the rather fluid Limes Arabicus. The Romans had built fortifications along the desert frontier here, but as I've emphasized, there was never a clear or official 'border' other than where the Romans were willing and able to control.
Now, Roman authority extended to Dumatha (Dumat al-Jandal, Al-Jawf) and Hegra due to the annexation of the Nabataeans. The Nabataean Kingdom was a client kingdom of Rome, but after the last king died, Rome ended up invading and annexing the kingdom in 106-107, and incorporated it into Provincia Arabia Petræa. Why would Rome want this territory? Possibly to gain better control over the area, to better control the Arabian trade routes which passed through the area, or some other reasons.
Fyi, it says on this post that it has "6 comments" but I can only see 3 for some reason. The bot's, Ameisen's and my own. So if you're reading this, you made a comment here and I didn't reply or anything it's probably because I can't see it.