Serving under Ursicinus he participated in various military actions in both the eastern and the western part of the Empire. After overthrowing the Frank usurper Silvanus at Gaul, Ursicinus was given temporary command of the Gallic armies. But in 357 Ursicinus and his staff were already sent back to the East because of the Persian threat. Ammianus tells us that the higher staff officers were given field command while the younger, among them Ammianus, had to carry out special commissions at short notice (16.10.21).
In 359 the siege of Amida took place, to which most of Book 19 is devoted. Ammianus' description of this event is one of the finest narratives in the Res Gestae. Whether Ammianus left the army when Ursicinus was dismissed after the fall of Amida, is unclear. Klein, Ensslin and Seeck, in their respective publications, do think so, but Matthews (1994) doesn't agree. It seems to him unlikely for a soldier at that time to leave and rejoin the army when the whim takes him. Furthermore Matthews mentions a letter of Libanius, dated 360, mentioning a certain Ammianus as a stratiotès (Epist. 233).
Ammianus probably did not serve on the staff of Ursicinus' successor Agilo, but we do find him on Julian's Persian expedition in 363. Of Ammianus' account of this campaign parts are written in the first person. Ammianus developed a profound admiration for Julian. His history at times comes close to a panegyric when writing about the pagan Augustus. Ammianus, himself being a pagan, supports the efforts of Julian trying to restore the traditional religious beliefs. However, this does not mean that Ammianus is uncritical towards his hero. When Julian e.g. forbids Christian teachers to teach, Ammianus considers this measure too extreme (25.4).
After the failure of the Persian expedition and the death of Julian, Ammianus seems to have definitely left the army. His whereabouts between his retirement from the army and his arrival at Rome around 378 can only be surmised. After the traumatic peace treaty of 363, Ammianus probably went to Antioch with the newly pronounced Emperor Jovian. After this he presumedly continued to travel and was able to see several parts of the Empire. Concluding from the long and detailed digressions and the use of the first person, he probably went to Egypt, the Black Sea, Thrace and Greece, where he saw the effects of the great earthquake of 366. A ship had been hurled two miles inland by an enormous tidal wave (26.10.19). He was also present at Antioch during the terror of the treason trials of 371-372 (29.1.24 ff).
Shortly after 378 Ammianus left for Rome. Here he dedicated the rest of his life to writing his Res Gestae. Beginning where Tacitus had left off in A.D. 96 Ammianus ended his narrative in 378 with the battle of Adrianople. He wrote his history using mainly written sources, but also his own experience - he had seen a lot of the Empire while in the army and as a traveller - as well as eyewitness' accounts.
Although he loved the city of Rome (16.10.15) his conceived image of it turned out not to correspond fully with reality. Ammianus dedicated two long digressions to the vices of the Roman population (14.6 and 28.4). His image was further shattered when in 383 he seems to have been ejected from the city along with other foreigners because of a famine. Ammianus writes with horror about the incident; 3000 dancing girls and other entertainers were allowed to stay (14.6.19)! >>>