It was by this time well known to the Romans that there were two, and two only, convenient roads whereby Southern Mesopotamia was to be reached, one along the line of the Mons Masius to the Tigris, and then along the banks of that stream, the other down the valley of the Euphrates to the great alluvial plain on the lower course of the rivers.

Julian had, perhaps, hitherto doubted which line he should follow in person.

The first had been preferred by Alexander and by Trajan, the second by the younger Cyrus, by Avidius Cassius, and by Severus.

Both lines were fairly practicable; but that of the Tigris was circuitous, and its free employment was only possible under the condition of Armenia being certainly friendly.

If Julian had cause to suspect, as it is probable that he had, the fidelity oL the Armenians, he may have felt that there was one line only which he could with prudence pursue.