His successor, Philip, was glad to make peace on any tolerable terms with the Persians; he felt himself insecure upon his throne, and was anxious to obtain the Senate's sanction of his usurpation.

He therefore quitted the East in A.D.244, having concluded a treaty with Sapor, by which Armenia seems to have been left to the Persians, while Mesopotamia returned to its old condition of a Roman province.
The peace made between Philip and Sapor was followed by an interval of fourteen years, during which scarcely anything is known of the condition of Persia.

We may suspect that troubles in the north-east of his empire occupied Sapor during this period, for at the end of it we find Bactria, which was certainly subject to Persia during the earlier years of the monarchy, occupying an independent position, and even assuming an attitude of hostility towards the Persian monarch.

Bactria had, from a remote antiquity, claims to pre-eminence among the Aryan nations.

She was more than once inclined to revolt from the Achaemenidae; and during the later Parthian period she had enjoyed a sort of semi-independence.
It would seem that she now succeeded in detaching herself altogether from her southern neighbor, and becoming a distinct and separate power.
To strengthen her position she entered into relations with Rome, which gladly welcomed any adhesions to her cause in this remote region.
Sapor's second war with Rome was, like his first, provoked by himself.
After concluding his peace with Philip, he had seen the Roman world governed successively by six weak emperors, of whom four had died violent deaths, while at the same time there had been a continued series of attacks upon the northern frontiers of the empire by Alemanni, Goths, and Franks, who had ravaged at their will a number of the finest provinces, and threatened the absolute destruction of the great monarchy of the West.