The pomerium was Rome’s sacred boundary that surrounded the city and ritually identified it as a domestic or civilian sphere, called domi or „at home“. In doing this, it differentiated and separated the city within its circuit from the rest of the world, the sphere militiae, where Rome’s armies engaged in the bloody business of war and conquest. In dividing the world into these two spheres, the pomerium also differentiated the types of authorities that Roman magistrates used in each: outside the pomerium in the sphere militiae, military commanders could employ the full force of their military authority (imperium) to give orders and compel obedience from otherwise free citizen-soldiers, but such untrammeled use of military authority was usually forbidden inside the pomerium, where citizens normally enjoyed the right of appeal from magisterial authority. Whenever a consul or praetor crossed the pomerium in either direction, therefore, the nature of his authority could change.

In Crossing the Pomerium: The Boundaries of Political, Religious, and Military Institutions from Caesar to Constantine, Michael Koortbojian explores different ways the Romans expressed the relationship between the pomerium and the emperor’s authority. While the title suggests a broad chronological survey of this relationship, the four chapters tend to focus on specific topics and emperors: Augustus receives most of the attention in the first two chapters, while styles of military dress and religious rites dominate the third chapter, and Constantine the fourth. Furthermore, Koortbojian is particularly interested in how the relationship between authority and the pomerium was expressed visually in various types of iconography (especially statues, coins, and monuments), although texts are also considered. For these reasons, the author rightly refers to his chapters as studies that “each in its own way” explores how the Romans understood the crossing of the pomerium (p. 9).

After a brief Introduction that explains the origins of the pomerium and lays out broad questions, the first two chapters examine ways that Caesar and Augustus changed the relationship between the imperator and the pomerium. The first chapter, “The Armed Ruler at Rome,” argues that Caesar and (later) Augustus both used their unusual statuses to set up statues of themselves inside the pomerium dressed in armor (statuae loricatae), which Koortbojian argues were a visual challenge to the traditional prohibition on military authority within the pomerium. He does suggest (p. 12) that armed soldiers were not unusual in the city, and he allows (p. 18) that other statuae loricatae may have been set up inside the city previously, but he argues that these statues of Caesar and Augustus were intended to make potent visual statements that, unlike other commanders, they were able to cross the pomerium and wield military authority inside the city. Chapter Two, “Octavian’s Imperium Auspiciumque in 43 BC and their late Republican Context,” examines the unusual nature of the emperor’s imperium and auspicium, and how they were exempt from the influence of the pomerium, since Augustus was allowed full use of both types of authority both domi and militiae. The chapter summarizes Republican traditions for conferring imperium and taking auspices, including discussions of the lex curiata and the auspices a commander took when leaving the city, to show how Augustus’ authority was different in that it was not changed by crossing the pomerium.

These first two chapters explore important questions about the changing nature of authority during the transition from Republic to Empire, and in doing so they engage in a particularly active scholarly debate. Few topics have provoked as much discussion as the natures of imperium, auspicium, and the pomerium in the Republic, and not much in these debates can be taken as settled. Koortbojian refers to many of these scholarly controversies in these chapters, but he generally keeps his engagement with them brief. He therefore attempts to strike a balance between referencing scholarly debates without allowing them to dominate his discussion, but some readers may feel that this leaves open questions about the natures of imperium, auspicium, and the pomerium before Caesar and Augustus started manipulating them.

The third chapter, “Roman Sacrifice and the Ritus Militaris,” makes excellent use of visual evidence to argue for a new interpretation of Roman ritual practices. It is generally understood that the Romans used their own ritual practice (ritus Romanus), characterized by the covering of the performant’s head, to worship some gods, while for other gods they used Greek ritual (ritus Graecus), in which the performant keeps his head uncovered. Koortbojian argues that the Romans normally depicted Greek ritual being used in military contexts, with the performant portrayed in military dress (paludatus), and so suggests that this should be understood as a form of military ritual (ritus militaris), whereas sacrifices performed for civic purposes employed the traditional Roman ritual. Thus the purpose of the sacrifice, rather than its location (domi or militiae), determined the type of ritual used, so that the ritus militaris could be used for certain sacrifices inside the pomerium as well as outside. Koortbojian makes clear that there were exceptions or variations in the portrayal of the separation of these ritual spheres, suggesting the distinction was normative but not necessarily normal (p. 115), and that it eventually broke down either through abandonment or social forgetting (p. 122).

The final chapter, “Constantine’s Arch and his Military Image at Rome,” looks closely at visual imagery and what it says about the emperor’s role in Rome and in the wider empire. Koortbojian argues that, despite the inclusion of the phrase arcum triumphis insignem on the arch, Constantine did not celebrate a triumph following his defeat of Maxentius in AD 312. Although the imagery on the arch is suggestive of a triumph, he argues that it lacks expected artistic elements of triumphal depictions, and it contains important variations, such as the emperor sitting in a carucca rather than standing in a quadriga. He suggests that the arch was meant to convey the idea that Constantine had celebrated a triumph, but it avoided portraying him as triumphing over Maxentius because triumphs over fellow citizens were not acceptable. The chapter examines a range of images to argue that the presentation of Constantine dressed in military garb in civilian settings (the Forum, the Senate) shows that the distinction between domi and militiae no longer existed as a political reality, and that Constantine’s iconography emphasized his military authority everywhere in the empire, including inside the pomerium. The book ends with a bibliography and several indices.

Crossing the Pomerium is a noteworthy achievement. It is not a book specifically about the pomerium, but about different ways the Romans expressed (especially visually) the idea of crossing the pomerium, and so moving from one sphere of authority into another. For this reason, it not only pays attention to the actual legal realities involved in an imperator crossing the pomerium, but also to the „fictions“ that were employed in the imagery of such crossings, which attempted to convey ideas that might not have been literally true. This theme of conceptual fictions is used many times (p. 32; 62; 90–101; 141), and provides a valuable way of understanding how seemingly concrete divisions of spheres could gradually break down, first in popular thought, and later in reality. The book will be broadly engaging to a wide audience, in particular because of the care and precision with which a large body of imagery is analyzed to reveal how the Romans thought about ancient and often-abstract ideas. To that end, the book contains sixty-two beautifully reproduced images on large pages that are easy to study. This interdisciplinary approach is very engaging and successful, and will no doubt be of great interest to scholars in a wide range of fields.