The Romans had some setbacks against the Parthians, imo related to the battle tactics of the Parthians and general fighting style of the Romans, who generally prevailed with astonishingly little manpower.
Trajan, probably the greatest general who ever became Emperor (and that’s saying something) kicked the living hell out of the Parthians, seized their capital, and added the Roman province of Mesopotamia.
For one brief shining moment (about three years) the Roman Empire stretched from Scotland (and Ireland, but that’s another story) and Denmark (ditto) to the Persian Gulf and Arabia and much of the length of modern Egypt.
Alas, Trajan died before he could launch what would have been the final campaign against the Parthians, and was succeeded by his catamite-loving allegedly adopted son Hadrian, who abandoned Mesopotamia, retreated behind some long walls in Britain and central Europe, and squandered Roman resources on lame-brained projects like Antinoos in Egypt. He spent nearly his entire reign touring the Empire, and allegedly knew every soldier in the Roman army by name (that story is obviously horse dung).
Hadrian had to be talked out of abandoning Dacia — had he gone ahead with it, the Roman Empire would never have had Aurelian in the 3rd century (he was born in Dacia), who, despite a mere five years as Emperor, reunited the Empire by eliminating all the rival sub-Caesars, smashed barbarian invasions, built the first city wall around Rome (it can still be seen here and there today), and consolidated Roman control in (you guessed it) the east. He was murdered by some embezzler on his staff who was afraid of being found out and punished.
Hey, I was just checking dates, and find that the Wikipedia page on Aurelian is pretty close to what I’d written. I just want to go on record that I wrote this without that, and didn’t write that. :’)
Actually, the story is better than that.
Aurelian was a harsh and suspicious ruler. With good reason, given recent history in the empire. He had a well-earned rep for executing corrupt or conspiring generals and officials.
So the embezzler you mention forged a list of names the emperor was planning to have executed, and went around showing it to the "intended victims."
Given Aurelian's character, they instantly believed the fraud and killed the emperor in what they thought was self-defense.
The moral being that suspicion and harshness can be every bit as deadly as the clemency that killed Julius Caesar.