“This book is an investigation of the process by which a man could become a Roman emperor.” So begins John Grainger’s The Roman Imperial Succession. Pen & Sword is best known as a military history publisher and that is likely a large part of the intended audience. It is a subject that Grainger has tackled before in part, with his 2003 work Nerva and the Roman succession crisis of AD 96-99. While the book does live up to Grainger’s opening statement, it may be better seen as a gateway to the topic than as the definitive, scholarly treatment of this important subject. First, I intend to give an overview of the content of his work, after which I will discuss his overall argument.
In his Introduction, Grainger notes that there is an accepted list of emperors and then other men who are deemed “usurpers,” but there is no clear definition that separates the one from the other. He further mentions the lack of any previous work that discusses in detail why some men are included on the “official” list while others are not. This is merely a lead-in to his own view, however, that the division may not have much importance. He states, rightly, that it can be a matter of subjective judgment as to who is an emperor and who is a usurper, and he further draws attention to the fact that men such as Augustus and Constantine began as usurpers but are accepted by all today as emperors. He prefers, therefore, to discuss successful claimants as opposed to failed ones, men who made an attempt to establish themselves as recognized Roman rulers but either ruled too briefly or were recognized in too small an area to be considered successes.
Grainger then examines the topic in a chronological fashion. The main body is divided into five Parts, each of which is further broken down into chapters. Each chapter is focused on a specific period of time, with especially perilous episodes in the chronicle of imperial successions being given the label “Crisis” (the first one being the so-called Year of the Four Emperors, which he discusses in Chapter 3 “The Crisis of 68-69”). Following such a crisis, the succeeding chapter is often titled “The Consequences” (i.e., for the figures involved in the preceding chapter) and discusses how the impact of the more fraught imperial accession during the crisis affected the handover of the office during the somewhat more stable period that followed.
Part I (“Augustus Defines the System”) consists of a single chapter devoted to the creator of the position. Grainger provides a very conventional account of the late Republic in which Augustus operated and then the creation of the set of powers, formal and informal, that made the “emperor” what he was. In Part II (“The Augustan Process”) he examines the period from the “First Imperial Family” until the reigns of the Severans in seven chapters. Three of these chapters are designated crises (Chapter 3 “The Crisis of 68-69,” Chapter 5 “The Crisis of 96-97,” and Chapter 7 “The Crisis of 193”), while the other four deal with periods in which the succession was somewhat less uncertain (the four “dynasty” periods, I might call them—Grainger would prefer not to do so, with the exception of the Flavians, to whom he refers with that word without resorting to placing quotation marks around it; this pattern of chapter titles will largely be followed in every Part). Again, the historical account he provides is largely conventional, giving due attention to the question of the succession and what was necessary for the handover of power to succeed.
The three chapters of Part III (“The Senate’s Revival”) document the crisis that accompanied the murder of Alexander Severus in 235 and the long period that has often been given the name “The Crisis of the Third Century”. After treating the obvious disconnect from prior practice that occurred during the reign of Maximinus Thrax in Chapter 9, where even the pretense of deference to the senate was thrown aside by a man who relied solely upon his control of the army, Grainger tackles the anarchic period of 238-284 in two chapters that look at “successful” and “unsuccessful” emperors separately. As he already signaled in his Introduction, he has cast aside the notion of “legitimate” vs. “usurping” emperors and instead focuses on men who were able to control parts of the Empire for significant periods of time and those who failed to do so. Therefore, it will possibly surprise some that Tetricus, the man who controlled the “Gallic Empire” is treated with the successful Roman emperors. In these chapters and those that follow, there is much more analysis offered, in comparison to the earlier part, which was more heavily dominated by the narrative.
In Part IV (“Heredity and Absolutism”) the five chapters cover Diocletian’s establishment of the Tetrarchy (Chapter 12) and then the undoing of that system by Constantine, who much more firmly embeds the hereditary principle into the nature of succession down to the sons of Theodosius. Grainger holds from the very start that heredity played a role in the succession, as Augustus intended to be succeeded by a man of his own blood, and it was only fate that prevented him from attaining that desire.
Though Grainger does not ignore events in Constantinople in Part V (“Breakdown”), the reign of Theodosius II removes the need to discuss succession in the East for a lengthy period. Accordingly, the final three chapters largely examine the situation in the West, taking events down to the “reign” of the last recognized emperor in the Western part of the Empire, Romulus Augustulus.
Finally, Grainger provides a summation of his arguments in a Conclusion. I will now discuss his argument in more detail. After the “constitutional contortions” (as he calls them earlier in the work) of Augustus, Grainger puts forward three essential elements that determined the succession to the imperial position: the previous emperor’s wishes, the senate, and the army. He notes the basic tension built into the process, as the man currently holding the position generally wished for it to be held by a blood descendent, preferably his own son, while the senate wanted the right to choose a man amenable to themselves, who would play the political game in Rome. The army was perhaps the most straightforward in its desires: they needed a commander, and it would be desirable if he was a generous paymaster as well. Heredity was not able to establish itself as the undisputed determining factor, as it has in most currently surviving European monarchies, largely owing to the lack of available direct male heirs in all but three cases (Vespasian, Marcus Aurelius, and Septimius Severus).
While current occupants successfully installed their chosen successors, with the senate more often serving to confirm the choice than having an active say in the matter, whenever there was no clear choice, the senators were able to have greater influence. In 96-97, Grainger argues that the senate overplayed its hand and was unable to prevent senior army commanders from forcing Nerva to accept Trajan as his successor. For the following period, until 238, the senate was a “ratifying body” and no more. The ruling emperor and the army (through its commanders) were the key players in deciding the succession. During the period of instability following Maximinus Thrax, down to the reign of Carus, the senate had some ability to affect the choice of emperor, though the army quickly knocked down emperors as soon as they were put up.
Diocletian largely removed the senate from the equation. He attempted to remove the army’s ability to choose the emperor as well, though his plan to have a system whereby competent generals could be promoted to the imperial dignity by the current holders was overturned, since two of his chosen successors, Constantius Chlorus and Maximian, had natural sons of their own. The widespread societal preference for hereditary succession undid the Tetrarchy. It would be the decisive factor in the choice of a successor from this point forward unless no satisfactory heir was available. In those cases, the basic elements involved in the succession during the time of Augustus remained largely unchanged into the final decades of the Roman Empire in the West, with the emperor in Constantinople holding the place of the senior emperor, and the senior magister militum or patrician in Italy representing the army, and the senate still attempting to be relevant.
The conclusions reached here will not occasion surprise for many. Grainger has rightly reduced the Praetorian Guard—much publicized, but largely ineffective—to the second tier of groups who influenced the system…if we can even dignify it with that name. Drawing attention to how many emperors died violent deaths and how often chaos threatened, Grainger does note that the “common factor in all of the murders and crises was the failure of the Roman governing system to devise an intelligible and workable system of imperial succession.” That failure to agree upon a system could do with further study. Further, Grainger has perhaps not taken sufficient notice of other factors that could influence the process, most notably the vast wealth and network of patronage that the emperor had at his disposal, allowing him to dispense with some rivals (through suborning informers) and buy off others, the better to smooth the path for his chosen heir.
In addition, I should point out that this work is likely not aimed only at a scholarly audience. The references, provided as endnotes, are limited in both number and scope: there is as much citation of primary sources as there is of modern scholarship. The bibliography runs to only four pages. In general, Grainger has consulted important monographs and some journal articles, but even delving briefly into the larger mass of scholarship turns up many items that touch upon the question of imperial succession (generally focused upon single succession episodes, not the subject as a whole) but find no place in either the notes or bibliography, and among the works listed, there are few items of recent date. But for a general introduction to the question of how one becomes a Roman emperor, Grainger has provided a sound guide.
 In his endnote 1 on p. 307, he provides a few works related to the issue, but none that discuss the entirety of it directly.
 P. 301.