Julian felt that, to attack Ctesiphon with success, he must, like Trajan and Severus, transport his army to the left bank of the Tigris, and deliver his assault upon the defences that lay beyond that river.

For the safe transport of his army he trusted to his fleet, which he had therefore caused to enter the Nahr-Malcha, and to accompany his troops thus far.

But at Coche he found that the Nahr-Malcha, instead of joining the Tigris, as he had expected, above Ctesiphon, ran into it at some distance below.

To have pursued this line with both fleet and army would have carried him too far into the enemy's country, have endangered his communications, and especially have cut him off from the Armenian army under Procopius and Sebastian, with which he was at this time looking to effect a junction.

To have sent the fleet into the Tigris below Coche, while the army occupied the right bank of the river above it, would, in the first place, have separated the two, and would further have been useless, unless the fleet could force its way against the strong current through the whole length of the hostile city.