Explanation: One of the recurring tactics of the Persian dynasties which Rome often warred with was simply to withdraw behind natural barriers inside of the Persian Empire. Roman forces would generally be unable to sustainably extend their supply lines, and thus either be forced to offer battle on Persian terms, or to withdraw.
The Roman Emperors Trajan and Septimius Severus both attempted to alleviate this weakness by annexing Mesopotamia, but in both cases, their successors found the expense of garrisoning and defending the province to be not worth the forward position. This was arguably "Penny-wise and pound-foolish", as Persian-owned Mesopotamia is exactly where Roman forces were regularly strung out and exhausted, but grand strategy is something that is not always fully appreciated by decision-makers in government, ironically - in antiquity especially, but in the modern day as well.