I would like to make a minor nitpick, Rome Conquered Britain in 43 AD under the Reign of Emperor Claudius a Mere 29 Years After Augustus' Death. They also conquered Dacia and Mesopotamia under Emperor Trajan in 104 AD some 90 years after Augustus' Death.
To Say that Rome Conquered nothing until Hadrian is patently false. . .
I did mention the British conquest elsewhere, but do notice I said
greatly. Rome's rate of conquest slowed immensely after the reign of Augustus, and following emperors largely kept to his strictures. Sure, Trajan conquered, but his conquests were rather rapidly reversed or abandoned by his successors, Hadrian among them.
Not just Agrippa, but Nero Claudius Drusus Germanicus, who avenged Rome's Defeat at Teutoberg Forest by utterly destroying Arminius' Confederation and recapturing the stolen Eagles
Germanicus only began the reversal against the Germans after Augustus' death. He stopped the legions in Gaul from revolting after Augustus died, then led them across the Rhine and recovered the legionary eagles. It's because of this success that led his uncle Tiberius to hate and fear him, and eventually kill him to secure his hold on the Empire.
Alright than what would we need to do to one day invade? Fund a rebelllion, they fought each other more times than they fought outsiders.
You could find a rebellion, but for an invasion of Parthia actually primarily led by yourself, you would need what Caesar had to invade Gaul:
1. Support in the Senate, or enough leverage/power to force it through.
2. Popularity with the people, and conditions that make the people amenable to a long and costly foreign war.
3. A pretext. This is easy enough — Rome, at times, used the diplomatic equivalent of a sneeze in their direction to justify subjugating whole nations. The Roman people love the myth of a 'just war' though, so you will need a
causus belli, no matter how flimsy.
4. Legions loyal enough to you personally to be willing to follow you into the graveyard of empires, or a military reputation great enough to command such respect.
5. Money. Caesar had Crassus and ungodly amounts of taxes, as well as atrociously large loans he had no intention of paying off. You need to feed, clothe, and pay the tens of thousands of men necessary to conquer Parthia.
6. Luck. Unlike Caesar, you will not be invading a bunch of divided tribes with deepseated rivalries that can be turned against each other, but a unified state with a semi-professional army capable of far better communication and cooperation than the Gauls. Caesar nearly lost when the Gauls rallied under a King — you will be facing a unified Persia from the start.
7. Time. It took Caesar a long time to conquer Gaul, and Persia is twice the size. You can just about multiply all the other requirements by two or three to account for the fact that you will be spending years, likely decades, away from your powerbase and allies.
Like I said, it is no easy thing to subdue Persia. It has never been.
Fits as a young second son looking to make his name and fortune, too.
