Notwithstanding the precautions which he had taken, Severus appears to have become straitened for supplies about the time that he captured Ctesiphon. His soldiers were compelled for some days to exist on roots, which produced a dangerous dysentery. He found himself unable to pursue Volagases, and recognized the necessity of retreating before disaster overtook him. He could not, however, return by the route of the Euphrates, since his army had upon its advance completely exhausted the resources of the Euphrates region. The line of the Tigris was therefore preferred for the retreat; and while the ships with difficulty made their way up the course of the stream, the army pursued its march upon the banks, without, so far as appears, any molestation. It happened, however, that the route selected led Severus near to the small state of Hatra, which had given him special offence by supporting the cause of his rival, Niger; and it seemed to him of importance that the inhabitants should receive condign punishment for this act of audacity. He may also have hoped to eclipse the fame of Trajan by the capture of a town which had successfully resisted that hero. He therefore stopped his march in order to lay siege to the place, which he attacked with military engines, and with all the other offensive means known at the time to the Romans. His first attempt was, however, easily repulsed. The walls of the town were strong, its defenders brave and full of enterprise. They burnt the siege-machines brought against them, and committed great havoc among the soldiers. Under these circumstances disorders broke out among the besiegers; mutinous words were heard; and the emperor thought himself compelled to have recourse to severe measures of repression. Having put to death two of his chief officers, and then found it necessary to deny that he had given orders for the execution of one of them, he broke up from before the place and removed his camp to a distance.