> By necessity, Augustus, therefore, set the boundaries of the Roman Empire at the Rhine and Danube rivers in Europe and negotiated a settlement with the Parthi

> By necessity, Augustus, therefore, set the boundaries of the Roman Empire at the Rhine and Danube rivers in Europe and negotiated a settlement with the Parthians in the East. The former proved durable almost until the end of the Roman Empire in the West, while the latter was the source of constant fighting for centuries in a series of conflicts known as the Roman-Persian Wars, which endured well into the Byzantine Era. > I think it’s plausible that Julius Caesar, who was on the verge of setting out for Parthia, would have either conquered the Parthians or dealt a serious blow that would have allowed the Romans to negotiate a much more favorable and stable Eastern border. What is interesting in the East, is that Rome's troubles in the East were worsened in a sense by their successes. Mesopotamia and Persia were initially ruled by the Seleucid Empire. Rome beat them up, and weakened them. Parthia was able to break away, and conquer much of the East. Rome tried to beat Parthia but was unable for a long time, until Trajan was able to actually attack and destroy the Parthian capitals. This ended up wearing the Parthians, which led to their downfall at the hands of Sassanids. This turned out to be really bad for Rome for two reasons. First, the Sassanids considered themselves the heirs of the orginal Achmenid Persian Empire and demanded that their former possessions be returned to them. Of course since this included the richest Roman provinces like Asia Minor, Syria, and Egypt it was a non-starter. Second, the Sassanid Capital was much further east than the Parthian Capital had been so was much harder for Rome to get to. So began centuries of Roman-Persian warfare, as mentioned which lasted into the Eastern Roman Empire (Byzantine) time period. It came to a culmination when the Byzantine Emperor Heraclius, did a brilliant attack against the rear of the Persian Army which was besieging Constantinople and brought the Persian Empire to its knees. This so weakened the Persian Empire, that just a short time later, they were conquered by the Muslim Arab armies of the Caliphate. The Caliphate would eventually take much of the remaining Roman(Byzantine) possessions in the East.