Why Wasn't Palmyra Invaded?

I understand that the Palmyrene Empire was responsible stabilizing the Sassanid border, but my question is why didn't the Sassanids invade the East a la Khosrau II?
 
Well they did in 260 (just before the establishment of the Empire), but were beaten back by Odenatus (husband of Zenobia). And considering that Palmyrene armies were strong enough to defeat Roman ones throughout the period, it was probably the wiser policy for Sassanid Persia to just sit back and wait for the two to fight to exhaustion, then reap the spoils.

We also don't really know if Shapur I had to deal with enemies on his Central Asian front, but considering that he founded Nishapur near the modern Iranian frontier, it's reasonable to assume that he would also busy with steppe invasions.
 
Top