Pax Romana is a comprehensive study of the Roman imperium through its phases of expansion and stability. Rome forms the model for our current understanding of imperial power, and the Romans were masters of both hard and soft power, going from one Iron Age city-state among many to unquestioned masters of the known world.

Roman expansion was driven by several related factors. The simplest was strength of arms. Roman legions were better drilled, better supplied, and had higher morale than comparable Mediterranean heavy infantry forces, leading to victories over their Italian neighbors, Gallic and Celtic tribes to the north, Seleucid Greeks to the east, and finally the Carthaginians.

Hard power achieved victories, but Roman soft power made them durable. Roman citizenship was surprisingly flexible and expansive, and granted to conquered peoples and allies. Latin, Roman dress and items, and Roman religion were prestigious, capturing local aristocratic cultures.

The third factor in expansion was the very idiosyncratic Roman republican political system, which in the words of a contemporary chronicler combined the best features of democracy, despotism, and aristocracy. Consuls and governors had the entire weight of the state behind them, but typically only a year in office, encouraging aggressive action by leaders posted to the frontier to demonstrate their accomplishments. Until the chaos of the civil war, this ambition was mostly directed outwards. Once the Republic was replaced by the Augustan principate, the borders became relatively fixed, and new provinces were rarely added.

Roman administration on the ground was surprisingly light. Senatorial governors and later Imperial legates travelled with small staffs, bolstered by friends and the ability to request support from the legions, who also served in variety of non-military roles. Cities were mostly governed under local laws, with Romans working with local magistrates. Cultural integration was a process of centuries.

Another aspect of this book is revolt and invasion. Many provinces experienced a revolt a generation or so after incorporation, which was invariably bloodily suppressed. Only Germany east of the Rhine was successful in breaking away. The province of Palestine was remarkable in revolting multiple times and being highly documented in doing so, given the relationship between Roman Jews and later Christianity, and conflicts between ardent monotheism and usually flexible imperial cult.

And finally, while the Roman border was very real, it was not exactly a rigid line as we might expect from modern borders. A military province, such as Germany, would have a line of forts and supporting settlements, a natural barrier such as a river, a depopulated zone, and then layers of allied tribes and kinds external to the border. The primary role of the border was to deter raiding, which was destructive to lives, property, and prestige. Even the imperial army had difficulty preventing all raids, but killing raiders as they retreated with their loot and organizing retaliatory raids was within the capabilities of distributed frontier forces. There was no strategic reserve in a modern sense. Romans expected that the time taken to gather up a major invading force would allow time for the legions to form up and meet in open battle, an expectation that held true into the 5th century.