For fifty years after Augustus’s death in 14 C.E. the dynasty that he established — known as the Julio-Claudians because all were members of the Julian and Claudian clans — provided the emperors of Rome. Some of the Julio-Claudians, such as Tiberius and Claudius, were sound rulers and created a bureaucracy of able administrators to help them govern. Others, including Caligula and Nero, were weak and frivolous.

In 68 C.E. Nero’s inept rule led to military rebellion and widespread disruption. Two years later Vespasian (r. 69–79 C.E.), who established the Flavian dynasty, restored order. He also turned Augustus’s principate into a hereditary monarchy and expanded the emperor’s powers. During the brief reign of Vespasian’s son Titus, Mount Vesuvius in southern Italy erupted, destroying Pompeii and other cities and killing thousands of people. The Flavians (69–96 C.E.) paved the way for the Antonines (96–192 C.E.), a dynasty of emperors under whose leadership the Roman Empire experienced a long period of prosperity and the height of the pax Romana. Wars generally ended victoriously and were confined to the frontiers. Second-century emperors made further changes in government that helped the empire run more efficiently while increasing the authority of the emperor.

The Roman army also saw changes, transforming from a mobile unit to a much larger defensive force, with more and more troops who were noncitizens. Because army service could lead to citizenship, non-Romans joined the army willingly to gain citizenship, receive a salary, and learn a trade. The frontiers became firmly fixed and were defended by a system of forts and walls. Behind these walls, the network of roads was expanded and improved, both to supply the forts and to reinforce them in times of trouble.