Rome a threat?

Not all the time, sometimes they were even good friends, for example Sassanid king Yazdegerd I was really one of best friends of Romans.

During Yazdergerd's reign, the Romans faced perilous dangers: the Ostrogoths were plundering the Balkans, the Franks were in rebellion, a civil war was raging, and eastern provinces were in revolt. Far from taking advantage of this situation, Yazdegerd returned Roman Christian captives whom the Persians had rescued after routing an invading Hunnic army. His good will was so well known that Emperor Arcadius, seeing that enemies were at the gate and his only son, Theodosius, was merely an infant, appointed Yazdegerd in his will as the child's guardian, "enjoining upon him earnestly to preserve the empire for Theodosius by all his power and foresight." The Persian king wrote to the senate, accepting the charge and threatening war against any who should attempt to enter into conspiracy against the child, and "loyally observing the behests of Arcadius, he adopted and continued without interruption a policy of profound peace with the Romans, and thus preserved the empire for Theodosius"