Octavian negotiated a treaty with the Parthians for the return of all legionary standards lost by Crassus and Mark Anthony. Crassus was suitably avenged by Trajan and Marcus Aurelius on the battlefield. The arabs and seljuks can in no way be regarded as having anything to do with Crassus.
A fair point, I'll admit. I had forgotten about Octavian's treaties...
Still, if Friedrich Barbarossa could get away with using the justification in our own world, who's to say that a particularly grandstanding Emperor with Eastern ambitions wouldn't make use of it in this one? Surely those points wouldn't have been common knowledge at the time.