The Roman empire followed a highly distinctive approach to governance, particularly at the highest levels of imperial administration. While most ancient empires were hereditary monarchies in which power typically passed from father to son, Rome initially diverged from this norm. The direct transmission of imperial authority from father to son did not become established practice until nearly a century after the empire’s foundation in 27 BCE, when the emperor Vespasian transferred power to his son Titus in 79 AD. Roughly a century later, Marcus Aurelius likewise succeeded in passing the throne to his biological son, Commodus, an exceptional case in Roman history. Notably, Marcus Aurelius had earlier shared power with Lucius Verus, ruling as co-emperors between 161 and 169, though this arrangement did not entail separate imperial courts or capitals. A more structured system of co-rule began under Diocletian between 285 and 293, culminating in the Tetrarchy of two Augusti and two Caesares. However, it collapsed soon after Diocletian’s abdication in 305. From Constantine’s acclamation in 306 until his sole rule in 324, the empire saw multiple emperors or claimants. Even after 324, Constantine shared power nominally with his sons and nephews. These power-sharing arrangements were marked by frequent conflict, particularly following Constantine’s death in 337, sparking a violent succession crisis and ongoing instability for the next two decades. The period of Constantine’s sons (337-361) is well-documented through classical and church historians, writings of key figures, and a wealth of legal, numismatic, epigraphic, and papyrological evidence.
William Lewis’s monograph examines the power struggles that occurred among Constantine’s sons and relatives, with Constantius II, his middle son, emerging as the central figure. Constantius II, who ruled for 24 years, was criticized by contemporaries like Ammianus Marcellinus for cruelty and poor judgment. Gibbon’s comparison of “the sons” to the politically shrewd Athanasius, further diminished his reputation as well. The introductory chapter, titled ‘Partes regendae, addresses the tradition of collegial rule within the Roman imperial administration. The author notes that the practice of power-sharing dates back to the early empire; however, he gives only little attention to the joint reign of Marcus Aurelius and Lucius Verus (161-169). In this section, Lewis outlines a conceptual framework for evaluating imperial division in the fourth century, offering only a brief discussion of the Tetrarchy (285-305). He suggests three criteria for identifying true political division: territorial sovereignty, political autonomy, and functional separation of power. Using these criteria, Lewis highlights three main periods when imperial authority was meaningfully divided: 305-306 (between Constantius Chlorus and Galerius), 313-324 (between Constantine and Licinius), and, to a lesser extent, 306-313, when several Augusti ruled at the same time.
Chapter Two, titled ‘Constantius, Divider of Empires?’, questions the dynastic purge that occurred in Constantinople during the early summer of 337. Lewis, who carefully traces the appointment of Caesars even prior to 324, emphasizes that those eliminated in the massacre included not only members of the imperial family, Julius Constantius, Flavius Dalmatius, and his sons Hannibalianus and Caesar Dalmatius, but also prominent court officials such as Flavius Optatus and Ablabius. He offers a compelling interpretation, suggesting that the violence may have been part of a broader effort to dismantle a faction that had begun to unite in the final years of Constantine’s reign, a group perceived as a potential threat to the authority of Constantius II, who had served as Caesar in the east since 335. Lewis cautions that the historiography surrounding this massacre is polarized, shaped both by hostile and sympathetic portrayals of Constantius. Nevertheless, the available evidence suggests that Constantine, by appointing Dalmatius as Caesar, may have envisioned a revived tetrarchic arrangement for the succession. In this design, his eldest sons, Constantine II and Constantius II, would have served as senior Augusti, while his younger son Constans and his nephew Dalmatius would have held junior positions as Caesars in the western and eastern regions, respectively. As the author himself prudently acknowledges, the extent to which this reconstruction reflects the actual political intentions of the time remains uncertain.
Chapter 3 examines the formal division of the empire among Constantine’s three sons on 9 September 337 and the establishment of a new imperial triumvirate. A central question is whether this arrangement, lasting only three years, and the broader distribution of administrative responsibilities between 337 and 361, should be interpreted as a true “division of the empire” or merely as a pragmatic “sharing of responsibilities.” According to the settlement reached in 337, the empire was partitioned as follows: Constantine II received Gaul, Spain, Britain, and Mauretania; Constans was assigned Italy, Africa, and Pannonia; and Constantius II retained the east. Moesia was allocated to Constans, while Thrace was assigned to Constantius. Despite this distribution, Constantine II was acknowledged as senior Augustus, and imperial officials across the empire could appeal directly to him. Lewis suggests that the return of Athanasius to Alexandria, having been exiled during the final years of Constantine I, was likely facilitated by Constantine II, in his capacity as senior emperor. Supporting this interpretation, Lewis draws attention to an inscription discovered in Thrace (p. 43, referring to Last Statues of Antiquity 1665), which he translates as evidence of Constantine II’s primacy. The alternating appointment of consuls from outside the imperial family in 338 and 340 is also read as a strategic manoeuvre, balancing expressions of collegial governance with individual assertions of authority.
Chapter 4, based on a previously published article by the author, investigates the civil war between Constantine II and Constans that culminated in 340. In this chapter, Lewis challenges the conventional narrative that characterizes Constantine II’s incursion into Italy as an unlawful act of aggression. He contends that the version of events promoted by Constans’ entourage, what Lewis terms the “official narrative”, obscures a more nuanced reality. Drawing upon lesser-known eastern sources of the conflict between Constans and Constantine II, primarily in Greek (like Libanius and Zonaras), Lewis argues that the latter may have had legitimate grounds for entering Italy and that he was, in fact, provoked or even ambushed by Constans. This reinterpretation is seen within a broader examination of factional dynamics, particularly those surrounding Constantius II’s court. Lewis highlights how this circle played a significant role in shaping a negative image of Constans, who increasingly diverged from Constantius in both ecclesiastical and political matters (in eastern Illyricum). Setting aside interpretive divergences, the outcome of the conflict is well known: Constantine II was killed near Aquileia in April 340, and Constans assumed control of the western portion of the empire.
Chapter Five concentrates on the western half of the empire during the reign of Constans, with particular attention given to Ulpius Limenius. One of the key contributions of this chapter lies in its effort to reintroduce the western provinces into the broader narrative of imperial administration. Limenius began his career as proconsul of Constantinople in 342 and subsequently held a series of high-ranking posts under Constans in the west, serving as consul at the time of his death in 349. His exceptionally rapid rise and accumulation of prestigious offices, across both eastern and western spheres of governance, signal a notable increase in political clout that is difficult to explain solely within traditional frameworks of advancement. Limenius’ trajectory suggests not only personal prominence but also an institutional continuity between the eastern and western courts. His career reflects a broader impulse toward integration rather than fragmentation within the empire. Moreover, Limenius was not an isolated case. Despite the mounting religious and political tensions between Constans and Constantius II, the rotation of senior officials between the two halves of the empire indicates that inter-imperial cooperation persisted. The mutual recognition and balancing of authority between the emperors suggest that, even amid ideological divergence, the administrative system retained a degree of internal cohesion and flexibility at its highest levels.
Chapter Six centres on the career of Fabius Titianus and explores the administrative developments that culminated in the collapse of Constans’s regime in 350. Lewis analyses the careers of urban prefects who served during Constans’s reign, drawing particularly on data from the Chronograph of 354. Among these figures, Titianus emerges as especially noteworthy due to his long and distinguished service: consul in 337, various provincial governorships, praefectus urbi from 339 to 341, praetorian prefect of Gaul from 341 to 349, and subsequently a key supporter of Magnentius, who was proclaimed emperor at Autun in January 350. In recognition of his role in facilitating the transfer of western authority to Magnentius, Titianus was reappointed as praefectus urbi between 350 and 351. Lewis interprets the continued appointment of individuals of pagan background to the post of the city prefect as a deliberate and pragmatic political strategy aimed at securing the loyalty of the senatorial elite. The case of Titianus exemplifies the complexities of aristocratic allegiance, demonstrating that claims to legitimacy vis-à-vis the Constantinian dynasty were shaped as much by competition within elite networks as by dynastic loyalty. Citing Zosimus (Hist. nov. 2.49.1-2), Lewis notes that Titianus had accused members of the Constantinian house of neglecting the cities, possibly as a rhetorical strategy to align himself with Constantius II. However, Titianus’s fortunes declined following Magnentius’s defeat in 353.
In Chapter Seven, Lewis extends the analysis beyond the year 350, offering a broader assessment of imperial unity and division throughout the rest of the fourth century. While his doctoral thesis had only briefly addressed this period, the present work offers a more detailed treatment. Following Magnentius’ defeat, Constantius II briefly reunited the empire under a single ruler. Nonetheless, the administrative and logistical burdens of sole rule prompted him to appoint Gallus, one of the few surviving members of the Constantinian family, as Caesar in 351, followed by Julian in 355. Gallus’ misrule led to his execution in 354, and Constantius himself died in 361 while preparing for a campaign against Julian. Julian’s own reign (361-363) was cut short during his Persian expedition, and his successor Jovian ruled for only a few months. The subsequent settlement of 364 saw the army elevate Valentinian I and his brother Valens as co-emperors, governing the west and east respectively. Valens ruled the east until his death at the battle of Adrianople in 378, while the western half became increasingly fragmented. Valentinian’s young son Gratian inherited power in 375, and following Valens’s death, Gratian appointed Theodosius as Augustus in 379. The power dynamics shifted once more in 383, when the military commander Maximus declared himself Augustus in Britain and orchestrated the assassination of Gratian. Theodosius intervened and defeated Maximus in 388, though authority in the western court soon passed to the Frankish general Arbogast. In 392, Arbogast took advantage of the death of Valentinian II and installed Eugenius as emperor, only to be defeated by Theodosius in 394. In a final attempt to ensure dynastic continuity, Theodosius raised his sons Arcadius (in 383) and Honorius (in 393) to the rank of Augustus. Upon Theodosius’s death in 395, the empire was formally divided between Arcadius in the east and Honorius in the west, a division that would persist and eventually solidify in the centuries to come.
The eighth and final chapter (pp. 196-210) considers the broader implications of the events analysed throughout the book, arguing that the reigns of Constantine’s sons established a lasting precedent for the political division of the empire in subsequent decades. Within this framework, the comparative table on pp. 202-203 maps the alternating periods of imperial unity and division, highlighting the varying degrees of administrative separation across time.
Rather than offering a straightforward chronological narrative of the reigns of Constantine’s sons, Lewis pursues a targeted analytical approach that foregrounds the interaction between administrative structures and political developments, a focus already signalled in the title of his book. His study presents a persuasive narrative of the fourth-century Roman empire as politically fragmented and administratively unstable, thus making a distinctive contribution to the political historiography of late antiquity. One of Lewis’s most notable achievements is his recovery of the political relationships and governing practices of Constantine’s sons, themes long obscured by the dominant religious agendas of the period. As the works of Timothy Barnes, reflected in Athanasius and Constantius, and much of the popular scholarship on late antiquity indicate, this period has traditionally been interpreted primarily through the lens of ecclesiastical or Christian history, with an emphasis on the eastern Mediterranean. Lewis demonstrates, however, that the period also possesses a rich administrative and political context, and he also shows how an analysis of prosopography can open new perspectives on imperial governance. To my knowledge, no modern study has examined the distribution and exercise of imperial authority during this period with comparable depth and precision.