character of the country, was able to maintain its ground without much difficulty; but that which had advanced by the line of the Euphrates and Tigris, and which was still marching through the boundless plains of the great alluvium, found itself suddenly beset by a countless host, commanded by Artaxerxes in person, and, though it struggled gallantly, was overwhelmed and utterly destroyed by the arrows of the terrible Persian bowmen.  Herodian says, no doubt with some exaggeration, that this was the greatest calamity which had ever befallen the Romans.  It certainly cannot compare with Cannae, with the disaster of Varus, or even with the similar defeat of Crassus in a not very distant region.  But it was (if rightly represented by Herodian) a terrible blow.  It absolutely determined the campaign.  A Caesar or a Trajan might have retrieved such a loss.  An Alexander Severus was not likely even to make an attempt to do so.  Already weakened in body by the heat of the climate and the unwonted fatigues of war, he was utterly prostrated in spirit by the intelligence when it reached him.  The signal was at once given for retreat.  Orders were sent to the corps d’ armee which occupied Media to evacuate its conquests and to retire forthwith upon the Euphrates.  These orders were executed, but with difficulty.  Winter had already set in throughout the high regions; and in its retreat the army of Media suffered great losses through the inclemency of the climate, so that those who reached Syria were but a small proportion of the original force.  Alexander himself, and the army which he led, experienced less difficulty; but disease dogged the steps of this division, and when its columns reached Antioch it was found to be greatly reduced in numbers by sickness, though it had never confronted an enemy.  The three armies of Severus suffered not indeed equally, but still in every case considerably, from three distinct causes—­sickness, severe weather, and marked inferiority to the enemy.  The last-named cause had annihilated the southern division; the northern had succumbed to climate; the main army, led by Severus himself, was (comparatively speaking) intact, but even this had been decimated by sickness, and was not in a condition to carry on the war with vigor.  The result of the campaign had thus been altogether favorable to the Persians, but yet it had convinced Artaxerxes that Rome was more powerful than he had thought.  It had shown him that in imagining the time had arrived when they might be easily driven out of Asia—­he had made a mistake.  The imperial power had proved itself strong enough to penetrate deeply within his territory, to ravage some of his best provinces, and to threaten his capital.  The grand ideas with which he had entered upon the contest had consequently to be abandoned; and it had to be recognized that the struggle with Rome was one in which the two parties were very evenly matched, one in which it was not to be supposed that either side would very soon obtain