In AD 46, at about age 23, Pliny entered the army as a junior officer, as was the custom for young men of equestrian rank.
Ronald Syme, Plinian scholar, reconstructs three periods at three ranks.
Pliny's interest in Roman literature attracted the attention and friendship of other men of letters in the higher ranks, with whom he formed lasting friendships.
Later, these friendships assisted his entry into the upper echelons of the state; however, he was trusted for his knowledge and ability, as well.
According to Syme, he began as a praefectus cohortis, a "commander of a cohort" (an infantry cohort, as junior officers began in the infantry), under Gnaeus Domitius Corbulo, himself a writer (whose works did not survive) in Germania Inferior.
In AD 47, he took part in the Roman conquest of the Chauci and the construction of the canal between the rivers Maas and Rhine.
His description of the Roman ships anchored in the stream overnight having to ward off floating trees has the stamp of an eyewitness account.

At some uncertain date, Pliny was transferred to the command of Germania Superior under Pomponius Secundus with a promotion to military tribune, which was a staff position, with duties assigned by the district commander.
Pomponius was a half-brother of Corbulo.
They had the same mother, Vistilia, a powerful matron of the Roman upper classes, who had seven children by six husbands, some of whom had imperial connections, including a future empress.
Pliny's assignments are not clear, but he must have participated in the campaign against the Chatti of AD 50, at age 27, in his fourth year of service.
Associated with the commander in the praetorium, he became a familiar and close friend of Pomponius, who also was a man of letters.

At another uncertain date, Pliny was transferred back to Germania Inferior.
Corbulo had moved on, assuming command in the east.
This time, Pliny was promoted to praefectus alae, "commander of a wing", responsible for a cavalry battalion of about 480 men.
He spent the rest of his military service there.
A decorative phalera, or piece of harness, with his name on it has been found at Castra Vetera, modern Xanten, then a large Roman army and naval base on the lower Rhine.
Pliny's last commander there, apparently neither a man of letters nor a close friend of his, was Pompeius Paulinus, governor of Germania Inferior AD 55-58.
Pliny relates that he personally knew Paulinus to have carried around 12,000 pounds of silver service on which to dine on campaign against the Germans (a practice which would not have endeared him to the disciplined Pliny).

According to his nephew,  during this period, he wrote his first book (perhaps in winter quarters when more spare time was available), a work on the use of missiles on horseback, De jaculatione equestri.
It has not survived, but in Natural History, he seems to reveal at least part of its content, using the movements of the horse to assist the javelin-man in throwing missiles while astride its back.
During this period, he also dreamed that the spirit of Drusus Nero begged him to save his memory from oblivion.
The dream prompted Pliny to begin forthwith a history of all the wars between the Romans and the Germans, which he did not complete for some years.
In AD 46, at about age 23, Pliny entered the army as a junior officer, as was the custom for young men of equestrian rank. Ronald Syme, Plinian scholar, reconstructs three periods at three ranks. Pliny's interest in Roman literature attracted the attention and friendship of other men of letters in the higher ranks, with whom he formed lasting friendships. Later, these friendships assisted his entry into the upper echelons of the state; however, he was trusted for his knowledge and ability, as well. According to Syme, he began as a praefectus cohortis, a "commander of a cohort" (an infantry cohort, as junior officers began in the infantry), under Gnaeus Domitius Corbulo, himself a writer (whose works did not survive) in Germania Inferior. In AD 47, he took part in the Roman conquest of the Chauci and the construction of the canal between the rivers Maas and Rhine. His description of the Roman ships anchored in the stream overnight having to ward off floating trees has the stamp of an eyewitness account.
At some uncertain date, Pliny was transferred to the command of Germania Superior under Pomponius Secundus with a promotion to military tribune, which was a staff position, with duties assigned by the district commander. Pomponius was a half-brother of Corbulo. They had the same mother, Vistilia, a powerful matron of the Roman upper classes, who had seven children by six husbands, some of whom had imperial connections, including a future empress. Pliny's assignments are not clear, but he must have participated in the campaign against the Chatti of AD 50, at age 27, in his fourth year of service. Associated with the commander in the praetorium, he became a familiar and close friend of Pomponius, who also was a man of letters.
At another uncertain date, Pliny was transferred back to Germania Inferior. Corbulo had moved on, assuming command in the east. This time, Pliny was promoted to praefectus alae, "commander of a wing", responsible for a cavalry battalion of about 480 men. He spent the rest of his military service there. A decorative phalera, or piece of harness, with his name on it has been found at Castra Vetera, modern Xanten, then a large Roman army and naval base on the lower Rhine. Pliny's last commander there, apparently neither a man of letters nor a close friend of his, was Pompeius Paulinus, governor of Germania Inferior AD 55-58. Pliny relates that he personally knew Paulinus to have carried around 12,000 pounds of silver service on which to dine on campaign against the Germans (a practice which would not have endeared him to the disciplined Pliny).
According to his nephew,  during this period, he wrote his first book (perhaps in winter quarters when more spare time was available), a work on the use of missiles on horseback, De jaculatione equestri. It has not survived, but in Natural History, he seems to reveal at least part of its content, using the movements of the horse to assist the javelin-man in throwing missiles while astride its back. During this period, he also dreamed that the spirit of Drusus Nero begged him to save his memory from oblivion. The dream prompted Pliny to begin forthwith a history of all the wars between the Romans and the Germans, which he did not complete for some years.
Pliny the Elder