Due to the lack of force of action of several Roman armies, Constantius was forced into diplomacy. Ammianus, being a soldier, could not appreciate that too much. When Constantius had to delay the renewed hostilities in the east in 359, he sent diplomats to the Persian king Sapor. Unfortunately, diplomacy did not work. While Constantius was with his army occupied in the west, Sapor saw his chance and sacked Amida (359), a fortified city in the east (19.1-9).
Ammianus was a competent jugde of military matters without seriously restricting his critical perspective. He therefore was able to assess the performance of all levels in the army, from the lowest ranks through the magistri to the emperors themselves (Crump 1975, 13). We have seen that Ammianus blames Constantius for celebrating his successes in civil wars, but that he fails to achieve victories in foreign affairs (21.16.15). On the other hand, he was careful in the maintenance of soldiers, critical in evaluating services. No person was appointed at a position for which he was unqualified (21.16.3). When king Sapor was giving the Romans a hard time, and Julian was appointed Augustus by his troops, Constantius had to act swiftly. He speed-marched his troops to the West to meet Julian. He died in Tarsus (3 Nov. 361) before the decisive battle with his cousin.
We have seen that Ammianus was a competent judge in military matters, but he also left us a description of the person behind the emperor Constantius.Constantius' personal characteristics according to Ammianus
Most of the characteristics are to be found in the obituary dedicated to Constantius (21.16). This obituary begins favourable. It states that Constantius was aware of the division of military and civil affairs. Constantius hereby obeyed ancient Roman laws. Further, dignity of imperial majesty was kept high, and Constantius was never obscene in public. He was athletic and kept a good and healthy condition. He also made great pretensions in learning, though he was in the end not the most eloquent man in the world (21.16.1-8).
There is in Ammianus an abundance of negative aspects to be found on Constantius. He is presented as an incompetent, malevolent, and vicious tyrant (cf. Barnes, 1998, 132). For one thing, Constantius raised high taxes. Even nowadays, this is not a too popular measure to be taken. Constantius needed the money, due to civil as well as foreign wars. But he was ruthless in raising and extracting taxes, the inexhaustible rapacity of his demands bringing him more unpopularity than money (21.16.17). Even though he was anxious to be known as a just and merciful emperor, he loosely pulled back exemptions of taxes, if he felt like it.
Then there is Constantius hunt for political opponents, and their trials. Like the opponents mentioned before, even his cousin and Caesar Gallus was recalled, arrested and executed (14.11.1-26). After that, a number of his followers were also arrested (15.3.1-6). Ammianus puts this in the following perspective: And as he [Constantius] deliberated with his closest friends, in secret conferences and by night, by what force or by what devices it seemed best that Gallus should be summoned [so that] he might be put to death without hindrance (14.11.1). Most striking is the use of the word nocturnis, or night. It indicates a sinister atmosphere of conspiracy, created by Ammianus, in which Gallus was taken prisoner and put to death.