The subjection of the Palmyrenes to the empire was, however, of a different nature to the ordinary one, and similar in some measure to the client-relation of the dependent kingdoms. Even in Vespasian’s time Palmyra is called an intermediate region between the two great powers, and in every collision between the Romans and Parthians the question was asked, what policy the Palmyrenes would pursue. We must seek the key to its distinctive position in the relations of the frontier and the arrangements made for frontier-protection. The Syrian troops, so far as they were stationed on the Euphrates itself, had their chief position at Zeugma, opposite to Biredjik, at the great passage of the Euphrates. Further down the stream, between the immediately Roman and the Parthian territory was interposed that of Palmyra, which reached to the Euphrates and included the next important place of crossing at Sura opposite to the Mesopotamian town Nicephorium (later Callinicon, now er-Ragga). It is more than probable that the guarding of this important border-fortress as well as the securing of the desert-road between the Euphrates and Palmyra, and also perhaps of a portion of the road from Palmyra to Damascus, was committed to the community of Palmyra, and that it was thus entitled and bound to make the military arrangements necessary for this far from slight task.[84] Subsequently doubtless the imperial troops were brought up closer to Palmyra, and one of the Syrian legions was moved to Danava between Palmyra and Damascus, and the Arabian legion to Bostra; after Severus united Mesopotamia with the empire, even here both banks of the Euphrates were in the Roman power, and the Roman territory on the Euphrates ended no longer at Sura but at Circesium, at the confluence of the Chaboras with the Euphrates above Mejadîn. Then Mesopotamia also was strongly occupied with imperial troops. But the Mesopotamian legions lay on the great road in the north near Resaina and Nisibis, and even the Syrian and Arabian troops did not supersede the need for the co-operation of the Palmyrenes. Even the protection of Circesium and of this part of the bank of the Euphrates may have been entrusted to the Palmyrenes. It was not till after the decline of Palmyra, and perhaps in compensation for it, that Circesium[85] was made by Diocletian a strong fortress, which thenceforth was here the basis of frontier-defence. [Sidenote: Administrative independence of Palmyra.]