Caesar invades Parthia

An ancient thread

but ironically, by adopting Seleucid arms Caesar could be very successful.  Incorporating Seleucid cataphracts, medium & light horse, mass Syrian archers and decent spearmen (not pikemen as that would probably be going too far :) ) would be the foundation for facing a steppe army.

Essentially evolving much like the Eastern Roman doctrine against cavalry armies of later date :)Though Caesar would be more successful, apart from simply being Caesar, because: 

1) Rome has much more manpower than in later eras, is much more vigorous, and 2) remember the Parthians are not like the Sassanids.

What matters is if there is the political will to go further afield.  Trajan handily swept through Mesopotamia and Elymais as the Seleucids did, retiring due to his age.  The Roman homeland is far to the west, whereas the Seleucids were based in Asia and thought of the Iranian domains as theirs.  If Caesar is willing, and evades the regularly scheduled mountain pass ambush after taking Media, he could go all the way.

As for the rebellions troubling the rear for which Hadrian pulled back from Trajan's conquests, at this early date the Parthians might not have a substantial hold on the local populations.  Local opinion and resistance may be significant.  Seleucus himself held Babylon & returned to Babylon from exile easily with the popular support of the people.