The heats of summer had arrived, and the malaria of autumn was not far off.

The stores brought by the fleet were exhausted; and there was a great risk in the army's depending wholly for its subsistence on the supplies that it might be able to obtain from the enemy's country.
Julian and his advisers must have seen at a glance that if the Romans were not to attack Ctesiphon, they must retreat.

And accordingly retreat seems to have been at once determined on.

As a first step, the whole fleet, except some dozen vessels, was burned, since twelve was a sufficient number to serve as pontoons, and it was not worth the army's while to encumber itself with the remainder.

They could only have been tracked up the strong stream of the Tigris by devoting to the work some 20,000 men; thus greatly weakening the strength of the armed force, and at the same time hampering its movements.