The fourth decade of the second century CE witnessed the outbreak and apex of the final Jewish uprising against Roman rule in Palestine. Named the Bar Kochba Revolt for its leader, its details remain shrouded in mystery. This chapter describes the revolt's direct causes, the geographical extent of Bar Kochba's regime and whether it included Jerusalem, and the magnitude of the Roman reaction. It also describes Bar Kochba's leadership style and administration, his state's borders, Jewish observance under wartime conditions, and the strong Roman reaction. Judaean desert documents provide a glimpse of Bar Kochba's administrative system. The list of Roman forces that participated in suppressing the Bar Kochba Revolt, compiled on the basis of epigraphic sources, assists in a tentative assessment of their magnitude. The Roman sense of having won a great victory emerges from Hadrian's second acclamation as imperator sometime after 135, following the revolt's suppression.

Any description of the eastern frontier must start with a discussion of the relationship between Rome and Persia. During most of the reign of Yezdegerd I (399-420) and in the first years of Theodosius II (408-50), relations between Rome and Persia were marked by a policy of mutual tolerance. In the fourth century the Arab nomad forces, 'Saracens', as they are called in the contemporary literature, became an important factor in the warfare between Rome and Persia to an extent previously unknown. The Jews in Palestine and the Diaspora had enjoyed a considerable degree of autonomy under Roman rule from the second century onward. The military organization of the second half of the fourth century had more in common with that of the sixth than with that of the third. This period witnessed the institution of territorial commands held by duces as distinct from the field arm.

The establishment of Roman rule in north Gaul can be seen archaeologically at central places like the great Treveran oppidum of the Titelberg in Luxembourg. The most important date for the establishment of the ideology of the new Caesar was the erection at Lugdunum of the Pan-Gallic altar, the Ara Galliarum, traditionally dated to 12 BC, the year of Agrippa's death. The earliest Romanizing tendencies revealed by the historical sources concern only the high Germanic nobility of the area between the Rhine and the Elbe. The overall command of Germanicus over the armies of both Upper and Lower Germany came to an end in AD16. The urbanization process continued in the Lower German military zone and in Gallia Belgica, proceeding from south to north, while in the Upper German military zone west of the Rhine there was no significant progress at all.