The army in Media, favored by the rugged 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.