The Flavians were succeeded by a line of relatively competent emperors known as the “five good emperors” — Nerva, Trajan, Hadrian, Antoninus Pius, and Marcus Aurelius. All of these emperors were experienced generals and members of the Senate, and they provided Rome with stable, effective political and military leadership for nearly a century, from 96 C.E. to 180 C.E.

Hadrian is typical of the emperors of the second century. His abilities and intelligence caught the attention of his elder cousin Trajan, the future emperor, who started him on a military career. At age nineteen, Hadrian served on the Danube frontier, where he learned the details of how the Roman army lived and fought and saw for himself the problems of defending the frontiers. When Trajan became emperor in 98 C.E., Hadrian was given important positions in which he learned how to defend and run the empire. Although Trajan did not officially declare Hadrian his successor, at Trajan’s death in 117, Hadrian assumed power.

Hadrian established more formal imperial administrative departments and separated civil service from military service. Men with little talent or taste for the army could instead serve the state as administrators. These innovations made running the empire more efficient and increased the authority of the emperor.

Under Trajan the boundaries of the Roman Empire were expanded to their farthest extent, and Hadrian worked to maintain most of these holdings, although he pulled back Roman armies from areas in the East he considered indefensible. No longer a conquering force, the army was expected to defend what had already been won. Forts and watch stations guarded the borders. Outside the forts the Romans built a system of roads that allowed the forts to be supplied and reinforced quickly in times of rebellion or unrest.

Roman soldiers also built walls, of which the most famous was one across northern England built primarily during Hadrian’s reign. Hadrian’s Wall, as it became known, protected Romans from attacks from the north and also allowed them to regulate immigration and trade through the many gates along the wall. Like all walls around cities or across territory, it served as a symbol and a means of power and control as well as a defensive strategy.

As the empire expanded, the army grew larger, and more and more troops were auxiliary forces of noncitizens. Because army service could lead to citizenship, men from the provinces and even from beyond the borders of the Roman Empire joined the army willingly to gain citizenship, receive a salary, and learn a trade. (See “Individuals in Society: Bithus, a Soldier in the Roman Army.”) The army evolved into a garrison force, with troops guarding specific areas for long periods.